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_arduino_mkrwifi1010.go

@@ -106,15 +106,9 @@ var (
 
 // SPI on the Arduino MKR WiFi 1010.
 var (
-	SPI0 = SPI{
-		Bus:    sam.SERCOM1_SPI,
-		SERCOM: 1,
-	}
+	SPI0 = sercomSPIM1
 
-	SPI1 = SPI{
-		Bus:    sam.SERCOM4_SPI,
-		SERCOM: 4,
-	}
+	SPI1     = sercomSPIM4
 	NINA_SPI = SPI1
 )