clk: renesas: r8a779a0: Use defines for PLL control registers

Add symbolic definitions for the various PLL control 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/8cac464c7dfb15ecd299b8ab4ba88a16135f8123.1721648548.git.geert+renesas@glider.be