sam: simplify SPI peripheral declaration

This has practically no effect on the resulting binaries, the only
difference I could find was for the flash/console/spi driver example.
I'm not sure how to test that one, but I think it's very unlikely that
code will have changed in any meaningful way (apart from reordering some
globals).

src/machine/board_grandcentral-m4.go

@@ -187,6 +187,12 @@ const (
 	SPI_CS_PIN  = SPI0_CS_PIN  //
 )
 
+// SPI on the Grand Central M4
+var (
+	SPI0 = sercomSPIM7
+	SPI1 = sercomSPIM2 // SD card
+)
+
 // I2C pins
 const (
 	I2C0_SDA_PIN = D62 // (PB20), also on D20