ARM: dts: silk: add sound support

Define the SILK board dependent part of the R8A7794 sound  device node.
Add device node for Asahi Kasei AK4643 stereo codec to the I2C1 bus.
Add the "simple-audio-card" device node to interconnect the SoC sound
device  and the codec.

This patch is based on the commit 493b4da7 ("ARM: dts: porter: add
sound support").

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>