mfd: si476x: Add header files and Kbuild plumbing

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: Samuel Ortiz <sameo@linux.intel.com>