spi/pxa2xx: convert to the common clk framework

Convert clk_enable() to clk_prepare_enable() and clk_disable() to
clk_disable_unprepare() respectively in order to support the common clk
framework. Otherwise we get warnings on the console as the clock is not
prepared before it is enabled.

In addition we must cache the maximum clock rate to drv_data->max_clk_rate
at probe time because clk_get_rate() cannot be called in tasklet context.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>