boards/nrf52dk: Arduino-compatible SPI config

Added a SPI config that allows using Arduino-Shields that use SPI via the
Arduino-ICSP header

boards/nrf52dk/include/periph_conf.h

@@ -28,6 +28,21 @@
 extern "C" {
 #endif
 
+/**
+ * @name    SPI configuration
+ * @{
+ */
+static const spi_conf_t spi_config[] = {
+    {
+        .dev  = NRF_SPI0,
+        .sclk = GPIO_PIN(0, 25),
+        .mosi = GPIO_PIN(0, 23),
+        .miso = GPIO_PIN(0, 24),
+    }
+};
+#define SPI_NUMOF           (sizeof(spi_config) / sizeof(spi_config[0]))
+/** @} */
+
 /**
  * @name    UART configuration
  * @{