sam: move I2S0 to machine file

There is no need to put these in the board files as the I2S is the same
on all Microchip SAM D21 chips. This simplifies the code and avoids some
special *_baremetal.go files.

This change does not change the resulting binaries.

src/machine/machine_atsamd21.go

@@ -897,6 +897,8 @@ type I2S struct {
 	Bus *sam.I2S_Type
 }
 
+var I2S0 = I2S{Bus: sam.I2S}
+
 // Configure is used to configure the I2S interface. You must call this
 // before you can use the I2S bus.
 func (i2s I2S) Configure(config I2SConfig) {