mfd: si476x: Add chip properties handling code

This patch adds code related to manipulation of the properties of
SI476X chips.

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>