ARM: shmobile: silk: add QSPI DT support

Define the SILK board dependent part of the QSPI device node.
Add device nodes  for Spansion  S25FL512S SPI flash and MTD partitions on it.

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

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>