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_circuitplay_express.go

@@ -96,6 +96,9 @@ const (
 	SPI0_SDI_PIN = PA16 // SDI: SERCOM3/PAD[0]
 )
 
+// SPI on the Circuit Playground Express.
+var SPI0 = sercomSPIM3
+
 // I2S pins
 const (
 	I2S_SCK_PIN = PA10