TARGET_NRF5: Corrected gpio_api to make P0.31 usable on NRF52.

hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/TARGET_NRF52_DK/PinNames.h

@@ -120,6 +120,7 @@ typedef enum {
     P0_28 = p28,
     P0_29 = p29,
     P0_30 = p30,
+    P0_31 = p31,
 
     LED1    = p17,
     LED2    = p18,

hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/gpio_api.c

@@ -20,7 +20,11 @@
 #include "nrf_drv_gpiote.h"
 
 
-#define GPIO_PIN_COUNT 31
+#if defined(TARGET_MCU_NRF51822)
+    #define GPIO_PIN_COUNT 31
+#else
+    #define GPIO_PIN_COUNT 32
+#endif
 
 typedef struct {
     bool         used_as_gpio : 1;