spi: mxs: implement runtime pm

This is a straight forward addition of runtime and system sleep pm operations
that handle clk and pinctrl (for runtime pm) and spi_master_{suspend,resume}
(for system sleep).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>