Add pinmap docs to target porting summary page

Add information on creating pinmaps for new targets in the porting
page.

Author: c1728p9

docs/porting/target/target.md

@@ -36,26 +36,42 @@ Add the target description to `mbed-os\targets\targets.json` using keys that the
 
 ### HAL porting
 
-There are many more APIs to implement. You enable the following APIs by adding a `device_has` attribute to the MCU_NAME target definition in `targets.json` and providing an implementation of the API declared in the API header.
-
-device_has       |   API header
------------------|------------------
-ANALOGIN         |   analog_in.h
-ANALOGOUT        |   analog_out.h
-CAN              |   can_api.h
-EMAC             |   emac_api.h
-INTERRUPTIN      |   gpio_irq_api.h
-I2C I2CSLAVE     |   i2c_api.h
-LPTICKER         |   lp_ticker_api.h
-LPTICKER         |   lp_ticker_api.h
-MPU              |   mpu_api.h
-PORT_IN PORT_OUT |   port_api.h
-PWMOUT           |   pwmout_api.h
-RTC              |   rtc_api.h
-SLEEP            |   sleep_api.h
-SPI SPISLAVE     |   spi_api.h
-TRNG             |   trng_api.h
-FLASH            |   flash_api.h
+There are many more APIs to implement. You enable the following APIs by adding a `device_has` attribute to the MCU_NAME target definition in `targets.json` and providing an implementation of the API declared in the API header along with supplying a pinmap.
+
+device_has       |   API header         |   Pinmaps
+-----------------|----------------------|------------------
+ANALOGIN         |   analog_in.h        | PinMap_ADC
+ANALOGOUT        |   analog_out.h       | PinMap_DAC
+CAN              |   can_api.h          | PinMap_CAN_TD/RD
+EMAC             |   emac_api.h         | N/A
+INTERRUPTIN      |   gpio_irq_api.h     | N/A
+I2C I2CSLAVE     |   i2c_api.h          | PinMap_I2C_SDA/SCL
+LPTICKER         |   lp_ticker_api.h    | N/A
+LPTICKER         |   lp_ticker_api.h    | N/A
+MPU              |   mpu_api.h          | N/A
+PORT_IN PORT_OUT |   port_api.h         | N/A
+PWMOUT           |   pwmout_api.h       | PinMap_PWM
+RTC              |   rtc_api.h          | N/A
+SLEEP            |   sleep_api.h        | N/A
+SPI SPISLAVE     |   spi_api.h          | PinMap_SPI_SCLK/MOSI/MISO/SSEL
+QSPI             |   qspi_api.h         | PinMap_QSPI_SCLK/SSEL/DATA0-3
+TRNG             |   trng_api.h         | N/A
+FLASH            |   flash_api.h        | N/A
+
+### Pinmap
+
+Pinmap information is added by creating a `PinMap` array which consists of a pin name, a peripheral and a function. The end of the pin map array is indicated by the presence of a NC pin. By convention a target's pinmap is defined in target specific file named PeripheralPins.c with a name matching that in the above table. An example of this for a UART TX pinmap is shown below:
+
+```C
+const PinMap PinMap_UART_TX[] = {
+    {P0_19, UART_0, 1},
+    {P1_13, UART_0, 3},
+    {P1_27, UART_0, 2},
+    { NC  , NC    , 0}
+};
+```
+
+Targets which don't make use of a pinmap, such as ones with peripherals that can be connected to any pin, must still define a pinmap as this is needed for testing. For these devices the pinmap does not need to be comprehensive. Instead it should list a representative sample of pins and peripherals so they can be tested appropriately.
 
 ### Testing