ASoC: si476x: Remove custom register I/O implementation

The current si476x I/O implementation wraps the regmap for the core with
functions that make the register map cache only when the device is powered
down. This implementation appears to be incomplete since there is no code
to synchronise the cache so writes done while the core is powered down
will be ignored, the device will only be configured if it is powered.

A better and more idiomatic approach would be to have the MFD manage the
cache, making the device cache only when it powers things down. This also
allows ASoC to use the standard regmap helpers for the device which helps
remove the ASoC custom ones so do convert to do that.

Signed-off-by: Mark Brown <broonie@linaro.org>