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_pygamer.go

@@ -2,8 +2,6 @@
 
 package machine
 
-import "device/sam"
-
 // used to reset into bootloader
 const RESET_MAGIC_VALUE = 0xf01669ef
 
@@ -111,12 +109,7 @@ const (
 )
 
 // SPI on the PyGamer.
-var (
-	SPI0 = SPI{
-		Bus:    sam.SERCOM1_SPIM,
-		SERCOM: 1,
-	}
-)
+var SPI0 = sercomSPIM1
 
 // TFT SPI pins
 const (
@@ -126,12 +119,7 @@ const (
 )
 
 // TFT SPI on the PyGamer.
-var (
-	SPI1 = SPI{
-		Bus:    sam.SERCOM4_SPIM,
-		SERCOM: 4,
-	}
-)
+var SPI1 = sercomSPIM4
 
 // USB CDC identifiers
 const (