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_matrixportal-m4.go

@@ -152,6 +152,14 @@ const (
 	NINA_SCK = SPI0_SCK_PIN
 )
 
+// SPI on the MatrixPortal M4
+var (
+	SPI0     = sercomSPIM3 // BUG: SDO on SERCOM1!
+	NINA_SPI = SPI0
+
+	SPI1 = sercomSPIM0
+)
+
 // HUB75 pins
 const (
 	HUB75_R1 = D7