spi: remove machine.SPI and replace with drivers.SPI interface for almost all SPI drivers

Signed-off-by: deadprogram <ron@hybridgroup.com>

spi.go

@@ -0,0 +1,7 @@
+package drivers
+
+// SPI represents a SPI bus. It is implemented by the machine.SPI type.
+type SPI interface {
+	Tx(w, r []byte) error
+	Transfer(b byte) (byte, error)
+}