clk: renesas: r8a77970: Add Z2 clock

Add support for the Z2 (Cortex-A53 System CPU) clock on R-Car V3M, which
uses a fixed SYS-CPU divider.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/6a9169e5bc92c2b9549292769a0814e04b9147cf.1676560357.git.geert+renesas@glider.be