clk: renesas: r8a77980: Add Z2 clock

Add support for the Z2 (Cortex-A53 System CPU) clock on R-Car V3H, which
uses a fixed SYS-CPU divider.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/aad9eaa57acf65cbe43e4d374066a72d760d54d8.1676560357.git.geert+renesas@glider.be