boards/adafruit-grand-central-m4-express: provide arduino features

The board is compatible with Arduino UNO and Arduino MEGA shields and
has an ISP connector, so this adds the corresponding features.

This adds the Arduino I/O-mapping for the ISP SPI and provides the
corresponding feature.

It appears that the SPI on D11/D12/D13 cannot be provided by a SERCOM,
this is under clarification at [1]. For now, no I/O mapping for that
SPI bus is provided.

[1]: https://forums.adafruit.com/viewtopic.php?t=214093

boards/adafruit-grand-central-m4-express/Makefile.features

@@ -17,4 +17,8 @@ FEATURES_PROVIDED += periph_usbdev
 # other board features
 FEATURES_PROVIDED += arduino_analog
 FEATURES_PROVIDED += arduino_pins
+FEATURES_PROVIDED += arduino_shield_isp
+FEATURES_PROVIDED += arduino_shield_mega
+FEATURES_PROVIDED += arduino_shield_uno
+FEATURES_PROVIDED += arduino_spi
 FEATURES_PROVIDED += sdcard_spi

boards/adafruit-grand-central-m4-express/include/arduino_iomap.h

@@ -85,6 +85,16 @@ extern "C" {
 #define ARDUINO_ANALOG_PIN_LAST 5
 /** @} */
 
+/**
+ * @name    Arduino's default SPI device
+ * @{
+ */
+/**
+ * @brief   SPI_DEV(0) is connected to the ISP header *AND* the SD card reader
+ */
+#define ARDUINO_SPI_ISP         SPI_DEV(0)
+/** @} */
+
 #ifdef __cplusplus
 }
 #endif

boards/adafruit-grand-central-m4-express/include/periph_conf.h

@@ -204,7 +204,7 @@ static const tc32_conf_t timer_config[] = {
  * @{
  */
 static const spi_conf_t spi_config[] = {
-    {   /* SPI on XIO connector */
+    {   /* SPI on XIO connector *AND* SPI on ISP */
         .dev      = &(SERCOM7->SPI),
         .miso_pin = GPIO_PIN(PD, 11),   /* D50 MISO */
         .mosi_pin = GPIO_PIN(PD, 8),    /* D51 MOSI */