[media] via-camera: specify XO-1.5 camera clock speed

For the ov7670 camera to return images at the requested frame rate,
it needs to make calculations based on the clock speed, which is
a completely external factor (depends on the wiring of the system).

On the XO-1.5, which is the only known via-camera user, the camera
is clocked at 90MHz.

Pass this information to the ov7670 driver, to fix an issue where
a framerate of 3x the requested amount was being provided.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>