ASoC: rsnd: use regmap_mmio instead of original regmap bus

Current rsnd driver is using regmap and regmap_field.
It used original regmap bus which is
single regmap instance for multi register mapping.
This patch modifies it to use regmap_mmio bus,
and tidyuped probe method

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>