ASoC: rcar: separate regmap init common field

The repmap initialization difference between Gen1/Gen2 is
only register offset.
This patch separates rsnd_gen1_regmap_init()
into common part and Gen1 specific part.

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