V4L/DVB (9462): Allow specifying clock per device

Optimizations galore: Blistering barnacles! The KNC1 and friends
like 90 Mhz clock much better rather than running at a higher
throttle, for almost similar hardware. he exact cause unknown,
possibly due to a lower voltage applied for the demod power supply.

Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>