mfd: si476x: Add commands abstraction layer

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