Describe the QSPI clock in the R8A7792 device tree.

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>