[media] omap3isp: Use the common clock framework

Expose the two ISP external clocks XCLKA and XCLKB as common clocks for
subdev drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>