spi: sh-msiof: Convert to clk_prepare/unprepare

Get the driver ready for the migration to the common clock framework by
calling clk_prepare() and clk_unprepare(). The calls are added in the
probe and remove handlers as the clk_enable() and clk_disable() calls
are located in atomic context and there's no callback function in
non-atomic context that can be used to prepare/unprepare the clock.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mark Brown <broonie@linaro.org>