ASoC: rsnd: share reg_field and reduce memory

Gen1/Gen2 code never be used in same time.
Thus, driver can share Gen1 only register and Gen2 only register.
It can reduce memory too.

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