ASoC: rcar: add rsnd_is_accessible_reg()

Current rcar driver is supporting Gen1,
and Gen2 will be supported soon.
Then, some registers are used from Gen1 only,
or from Gen2 only.
To avoid NULL pointer access, this patch adds
register accessible check function.

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