V4L/DVB (12243): em28xx: allow specifying sensor xtal frequency

In order to properly estimate fps, mt9v011 sensor driver needs to know
what is the used frequency on the sensor cristal. Adds the proper fields
and initialization code for specifying the cristal frequency.

Also, based on experimentation, it was noticed that the Silvercrest is
outputing data at 7 fps. This means that it should be using a 6.3 MHz
cristal. This information needs to be double checked later, by opening
the device. Anyway, by using this value for xtal, at least now we have
the correct fps report.

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