ARM: dts: r8a7794: add sound support

Define the generic R8A7794 part of  the sound device node.
This sound device  is a complex one and comprises the Audio Clock Generator
(ADG), Sampling Rate Converter Unit (SCU), Serial Sound Interface [Unit]
(SSI[U]), and Audio DMAC-Peripheral-Peripheral.
It is up  to the board file to enable the device.

This patch is based on the R8A7791 sound work by Kuninori Morimoto.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>