ARM: dts: r8a7794: remove Z clock

R8A7794 doesn't have Cortex-A15 CPUs, thus there's no Z clock...

Fixes: 0dce5454 ("ARM: shmobile: Initial r8a7794 SoC 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>