[media] mfd: Add header files and Kbuild plumbing for SI476x MFD core

This patch adds all necessary header files and Kbuild plumbing for the
core driver for Silicon Laboratories Si476x series of AM/FM tuner
chips.
The driver as a whole is implemented as an MFD device and this patch
adds a core portion of it that provides all the necessary
functionality to the two other drivers that represent radio and audio
codec subsystems of the chip.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>