ARM: dts: porter: add sound support

Define the Porter board dependent part of the R8A7791 sound device node.
Add device node for Asahi Kasei AK4642 stereo codec  to the I2C2 bus.
Add the "simple-audio-card" device node to interconnect the SoC sound
device  and the codec.

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>