ASoC: rsnd: Add device tree support for r8a774[35]

Document r8a774[35] specific compatible strings. The Renesas RZ/G1[ME]
(r8a774[35]) sound modules are identical to the R-Car Gen2 family.
No driver change is needed as the fallback compatible string
"renesas,rcar_sound-gen2" activates the right code in the driver.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>