spi/pl022: Unprepare clocks while suspended

When the driver was converted to clk_prepare() the suspend path didn't
have any changes made so the clock remains prepared throughout the runtime
of the driver. Unprepare it when suspended so that any savings that can be
made as a result are made.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>