ASoC: rsnd: Add device tree binding for r8a77990

This patch adds the device tree binding of the r8a77990 SoC.

Signed-off-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>