ARM: shmobile: r8a7778: add sound to DT

Adds an entry for an R-Car Gen. 1 sound controller.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
[horms: Removed undocumented "renesas,rcar_sound" compat string]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>