ARM: dts: r8a7792: add DU clocks

Describe the DU0/1 clocks and their parent, ZX clock in the R8A7792 device
tree.

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>