ASoC: rsnd: add RSND_GEN3 for R-Car Gen3

rsnd driver is supporting Gen3. The difference between Gen1 and Gen2
were very big, but, between Gen2 and Gen3 are not so much.
Thus, it is assuming Gen2 and Gen3 have compatible, therefore,
there is no RSND_GEN3 and rsnd_is_gen3() macro.
But in the future, it will need Gen2 and Gen3 different operation,
and for Gen4.
This patch adds missing RSND_GEN3 and rsnd_is_gen3() macro.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Nguyen Viet Dung <dung.nguyen.aj@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>