mtd: nand: pxa3xx: Use clk_prepare_enable and clk_disable_unprepare

This patch converts the module to use clk_prepare_enable and
clk_disable_unprepare variants as required by common clock framework.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>