[media] omap3isp: Move definitions required by board code under include/media

XCLK definitions are often required by the board code. Move them to public
include file.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>