media: cx231xx: Add second i2c demod client

Include ability to add a i2c device style frontend to cx231xx USB
bridge. All current boards set to use frontend[0]. Changes are
backwards compatible with current behaviour.

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>