[media] mfd: Add commands abstraction layer for SI476X MFD

This patch adds all the functions used for exchanging commands with
the chip.
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>