QSPI: add API and how to enable it

- use device_has to enable it
- add API to be ported

Author: 0xc0170

docs/reference/contributing/target/QuadSPI.md

@@ -7,7 +7,7 @@ QSPI HAL defines API for targets that contain QSPI capable peripheral. The QSPI
 ##### Defined behavior
 
 - a target implementaion should cover the most of QSPI frame format (some targets might not provide the flexibility for setting all frame parameters)
-- command transfer - a target might provide additional API for sending device specific commands. In case it does not, it can be implemented via read/write functions
+- command transfer - a target might provide additional API for sending device specific commands. In case it does not, it can be implemented via read/write functions (this is target/driver dependent)
 
 ##### Undefined behavior
 
@@ -19,9 +19,29 @@ QSPI peripheral
 
 #### Implementing QuadSPI
 
-The target needs to define `qspi_s` structure - target specific QSPI object, enable QSPI in targets.json file `device_has` and implement QSPI HAL functions defined in `hal/qspi_api.h` header file.
+The target needs to define `qspi_s` structure - target specific QSPI object.
 
-`qspi_write` and `qspi_read` are used for data transfers. For communicating with device, `qspi_command_transfer` should be used.
+Functions to implement:
+
+```
+qspi_status_t qspi_init(qspi_t *obj, PinName io0, PinName io1, PinName io2, PinName io3, PinName sclk, PinName ssel, uint32_t hz, uint8_t mode);
+qspi_status_t qspi_free(qspi_t *obj);
+qspi_status_t qspi_frequency(qspi_t *obj, int hz);
+qspi_status_t qspi_write(qspi_t *obj, const qspi_command_t *command, const void *data, size_t *length);
+qspi_status_t qspi_command_transfer(qspi_t *obj, const qspi_command_t *command, const void *tx_data, size_t tx_size, void *rx_data, size_t rx_size); 
+qspi_status_t qspi_read(qspi_t *obj, const qspi_command_t *command, void *data, size_t *length);
+
+```
+
+`qspi_write` and `qspi_read` are used for data transfers. For communicating with a device, `qspi_command_transfer` should be used.
+
+To enable QSPI HAL, define `QSPI` in targets.json file inside `device_has`:
+
+```
+"TARGET_NAME": {
+   "device_has": ["QSPI"]
+}
+```
 
 #### Testing