arm64: dts: r8a7796: Add external audio clocks

Add the external audio clocks as zero Hz fixed-frequency clocks.
Boards that provide these clocks should override them.

Based on commit 623197b9 ("arm64: renesas: r8a7795: Sound SSI
PIO support").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>