arm64: dts: renesas: r8a7796: add SSIU support for sound

rsnd driver supports SSIU now, let's use it.
Then, BUSIF DMA settings on rcar_sound,ssi (= rxu, txu) are
no longer needed.
To avoid git merge timing issue / git bisect issue,
this patch doesn't remove it so far, but will be removed in
the future.

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