ARM: dts: r8a7793: add audio clock to device tree

Instantiate audio clock in r8a7793 device tree.
audio_clk_a/b/c are required for R-Car sound.

Based on similar work for the r8a7791 by Kuninori Morimoto.

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