nrf5x Let application decide USBD_IRQn priority

hw/bsp/pca10056/board_pca10056.c

@@ -183,6 +183,8 @@ void board_init(void)
   nrfx_qspi_cinstr_xfer(&cinstr_cfg, &sr_quad_en, NULL);
 #endif
 
+  NVIC_SetPriority(USBD_IRQn, 2);
+
   // USB power may already be ready at this time -> no event generated
   // We need to invoke the handler based on the status initially
   uint32_t usb_reg;