[media] V4L: pxa_camera: add clk_prepare/clk_unprepare calls

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

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>