ARM: dts: wheat: add QSPI support

Define the Wheat 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 (and large) patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.

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