ASoC: si476x: Convert SI476X codec to use regmap

The latest radio and MFD drivers for SI476X radio chips use regmap API
to provide access to the registers and allow for caching of their
values when the actual chip is powered off. Convert the codec driver
to do the same, so it would not loose the settings when the radio
driver powers the chip down.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>