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_metro-m4-airlift.go

@@ -103,6 +103,15 @@ const (
 	SPI1_SDI_PIN = D13 // SCK:  SERCOM1/PAD[0]
 )
 
+// SPI on the Metro M4.
+var (
+	SPI0     = sercomSPIM2
+	NINA_SPI = SPI0
+)
+
+// SPI1 on the Metro M4 on pins 11,12,13
+var SPI1 = sercomSPIM1
+
 // USB CDC identifiers
 const (
 	usb_STRING_PRODUCT      = "Adafruit Metro M4 Airlift Lite"