ARM: dts: r8a7792: add QSPI support

Define the generic R8A7792 part of the QSPI device node.

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>