boards: adafruit: feather_m0_basic_proto: define feather_spi bus

Define the feather_spi SPI bus, which points to the corresponding
SERCOM. This bus label can be used by the featherwing family of
shields.

Signed-off-by: Philip-Dylan Gleonec <[email protected]>

Author: P-D-G

boards/adafruit/feather_m0_basic_proto/feather_connector.dtsi

@@ -6,3 +6,4 @@
  */
 
 feather_i2c: &sercom3 {};
+feather_spi: &sercom4 {};