[media] V4L2: em28xx: register a V4L2 clock source

Camera sensors usually require a master clock for data sampling. This patch
registers such a clock source for em28xx cameras. This fixes the currently
broken em28xx ov2640 camera support and can also be used by other camera
sensors.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>