ARM: dts: r8a7792: add PLL1 divided by 2 clock

Despite the fact that QSPI clock has PLL1/VCOx1/4 clock as a parent, the
latter hasn't been added to the R8A7792 device tree. This patch corrects
that oversight.

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>