spi/sirf: use clk_prepare_enable and clk_disable_unprepare

Convert clk_enable/clk_disable to clk_prepare_enable/clk_disable_unprepare
calls as required by common clock framework.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>