arm64: dts: renesas: r8a774a1: Add SSIU support for sound

Add SSIU support to the SoC DT as the sound driver supports
it now, and also since the sound driver can now handle
BUSIF0-7 via SSIU remove the no longer needed "rxu" and "txu"
properties.

Based on similar work from Kuninori Morimoto and Simon Horman in commits
8d14bfa0 ("arm64: dts: renesas: r8a7796: add SSIU support for
sound") and 10bd03fa ("arm64: dts: renesas: r8a7796: remove BUSIF0
settings from rcar_sound,ssi").

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>