ARM: dts: r8a7792: Correct Z clock

Unlike other R-Car Gen2 SoCs with Cortex-A15 CPU cores, R-Car V2H does
not have a programmable Z clock (Cortex-A15 CPU core clock), but uses a
fixed divider.
This is similar to the Z2 clock (Cortex-A7 CPU core clock) on R-Car E2.

Hence:
  - Remove the Z clock output from the cpg_clocks node, as this implied
    a programmable clock,
  - Add the Z clock as a fixed factor clock,
  - Let the first CPU node point to the new Z clock,
  - Remove the Z clock index from the bindings (this definition was used
    by r8a7792.dtsi only, and was not a contract between DT and driver).

Fixes: 7c4163aa ("ARM: dts: r8a7792: initial SoC device tree")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>