spi: rspi: Convert to clk_prepare_enable/disable_unprepare

Get the driver ready for the migration to the common clock framework by
calling clk_prepare_enable() and clk_disable_unprepare().

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>