ASoC: rsnd: Add device tree support for r8a77995

Simply document new compat strings.
There appears to be no need for a driver updates.

Signed-off-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>