ARM: dts: r8a7794: add audio clocks

Describe  the external audio clocks (signals AUDIO_CLK[ABC]) required by
the sound driver.  Boards that provide audio clocks need to  override the
clock frequencies.

This patch is based on the commit 0d3dbde8

 ("ARM: shmobile: r8a7791:
add  audio clock on DTSI").

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>