Mark volatile registers to avoid caching bugs.

Note: SPI_MCR is marked volatile because of CLR_TXF and CLR_RXF bits.
Signed-off-by: NEsben Haabendal <eha@deif.com>
Acked-by: NMartin Hundebøll <martin@geanix.com>
Signed-off-by: NMark Brown <broonie@kernel.org>