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_wioterminal.go

@@ -390,6 +390,20 @@ const (
 	SPI3_SDI_PIN = SDI3 // SDI: SERCOM7/PAD[2]
 )
 
+// SPI on the Wio Terminal
+var (
+	SPI0 = sercomSPIM5
+
+	// RTL8720D
+	SPI1 = sercomSPIM0
+
+	// SD
+	SPI2 = sercomSPIM6
+
+	// LCD
+	SPI3 = sercomSPIM7
+)
+
 // USB CDC identifiers
 const (
 	usb_STRING_PRODUCT      = "Seeed Wio Terminal"