media: saa7134: add P7131_4871 analog inputs

The saa7134 Tiger board has a front panel connector at the back (labeled
panel 2 on the PCB), with S-VIDEO, composite and audio.

This patch adds those inputs.

Signed-off-by: Alexandre GRIVEAUX <agriveaux@deutnet.info>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>