QSPI: add flash pins for nrf52480_dk board

Author: 0xc0170

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_NRF52840_DK/PinNames.h

@@ -227,6 +227,13 @@ typedef enum {
     A4 = p30,
     A5 = p31,
 
+    QSPI_FLASH_IO0 = P0_20,
+    QSPI_FLASH_IO1 = P0_21,
+    QSPI_FLASH_IO2 = P0_22,
+    QSPI_FLASH_IO3 = P0_23,
+    QSPI_FLASH_SCK = P0_19,
+    QSPI_FLASH_CSN = P0_17,
+
     // Not connected
     NC = (int)0xFFFFFFFF
 } PinName;