V4L/DVB (7006): cx23885: Track the board clock frequency and allow overrides

The cx23885/6/8 all have different clock rates, this patch allows
the core to compensate, and developers to allow vendor specific
overrides. This patches will be used by future analog video
and encoder patches.

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>