clk: renesas: rcar-gen4: Use defines for common CPG registers

Add symbolic definitions for common CPG registers.
Replace hardcoded register offsets by the new definitions.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/8ae48a5dac59cb5723fbca3842b93a9e51ffe1ca.1721648548.git.geert+renesas@glider.be