[media] saa7115: add support for double-rate ASCLK

Some devices expect a double rate ASCLK. Add a flag to let the driver know
through the s_crystal_freq call.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>