clk: renesas: r8a779g0: Add Z0 clock support

Add support for the Z0 (Cortex-A76 Sub-System) clock on R-Car V4H, based
on the existing support for Z clocks on R-Car Gen4.

Extracted from a patch in the BSP by LUU HOAI.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/75daa1fd8fa7eaef7b8945bb5906c787222c7ac4.1668423063.git.geert+renesas@glider.be