ARM: shmobile: r8a7791 dtsi: Drop "renesas,rcar_sound" compatible value

The "renesas,rcar_sound" compatible property value was never processed
nor documented.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>