ARM: shmobile: silk: initial device tree

Add the initial device  tree for the R8A7794 SoC based SILK low cost board.
SCIF2 serial port support is included, so that the serial console can work.

Based on the original patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>