ARM: dts: r8a7793: add R-Car sound support to device tree

Instantiate R-Car sound node in r8a7793 device tree.
This only supports PIO transfers.

Based on similar work for the r8a7791 by Kuninori Morimoto.

Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>