video: pxafb: add clk_prepare/clk_unprepare calls

This patch adds clk_prepare/clk_unprepare calls to the pxafb
driver by using the helper functions clk_prepare_enable and
clk_disable_unprepare.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@marvell.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>