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/machine_generic.go

@@ -182,4 +182,13 @@ var (
 	sercomI2CM5 = &I2C{5}
 	sercomI2CM6 = &I2C{6}
 	sercomI2CM7 = &I2C{7}
+
+	sercomSPIM0 = SPI{0}
+	sercomSPIM1 = SPI{1}
+	sercomSPIM2 = SPI{2}
+	sercomSPIM3 = SPI{3}
+	sercomSPIM4 = SPI{4}
+	sercomSPIM5 = SPI{5}
+	sercomSPIM6 = SPI{6}
+	sercomSPIM7 = SPI{7}
 )