mfd: si476x: Add the bulk of the core driver

This patch adds main part(out of three) of the I2C driver for the
"core" of MFD device.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>