Add resource management for serial for the NRF52 family

Instance counter keeps track of how many objects have been
initialized and freed. On the first object the instance is
enabled and on the last object the instance is disabled.

Author: Marcus Chang

targets/TARGET_NORDIC/TARGET_NRF5x/serial_api.c

@@ -188,6 +188,7 @@ typedef struct {
     uint8_t buffer[NUMBER_OF_BANKS][DMA_BUFFER_SIZE];
     uint32_t rxdrdy_counter;
     uint32_t endrx_counter;
+    uint32_t usage_counter;
     uint8_t tx_data;
     volatile uint8_t tx_in_progress;
     volatile uint8_t rx_in_progress;
@@ -1006,7 +1007,6 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
         /* Enable interrupts for UARTE0. */
         NVIC_SetVector(UARTE0_UART0_IRQn, (uint32_t) nordic_nrf5_uart0_handler);
         nordic_nrf5_uart_irq_enable(0);
-        nrf_uarte_enable(nordic_nrf5_uart_register[0]);
 
 #if UART1_ENABLED
         /* Initialize FIFO buffer for UARTE1. */
@@ -1019,7 +1019,6 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
         /* Enable interrupts for UARTE1. */
         NVIC_SetVector(UARTE1_IRQn, (uint32_t) nordic_nrf5_uart1_handler);
         nordic_nrf5_uart_irq_enable(1);
-        nrf_uarte_enable(nordic_nrf5_uart_register[1]);
 #endif
     }
 
@@ -1028,6 +1027,15 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
 
     uart_object->instance = instance;
 
+    /* Increment usage counter for this instance. */
+    nordic_nrf5_uart_state[instance].usage_counter++;
+
+    /* Enable instance on first usage. */
+    if (nordic_nrf5_uart_state[instance].usage_counter == 1) {
+
+        nrf_uarte_enable(nordic_nrf5_uart_register[instance]);
+    }
+
     /* Store pins in serial object. */
     if (tx == NC) {
 
@@ -1086,7 +1094,27 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
  */
 void serial_free(serial_t *obj)
 {
+    MBED_ASSERT(obj);
+
+#if DEVICE_SERIAL_ASYNCH
+    struct serial_s *uart_object = &obj->serial;
+#else
+    struct serial_s *uart_object = obj;
+#endif
+
+    int instance = uart_object->instance;
+
+    if (nordic_nrf5_uart_state[instance].usage_counter > 1) {
 
+        /* Decrement usage counter for this instance. */
+        nordic_nrf5_uart_state[instance].usage_counter--;
+
+        /* Disable instance when not in use. */
+        if (nordic_nrf5_uart_state[instance].usage_counter == 0) {
+
+            nrf_uarte_disable(nordic_nrf5_uart_register[instance]);
+        }
+    }
 }
 
 /** Configure the baud rate