V4L/DVB (12242): mt9v011: implement core->s_config to allow adjusting xtal frequency

Since frames per second is a function of cristal frequency, and this is
device-specific, add a function that allows adjusting it, via
subdev->core->s_config callback.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>