clk: renesas: sh73a0: Remove sh73a0_cpg.reg

The register block base pointer as stored in the reg member of the
sh73a0_cpg structure is only used during initialization.  Hence move
it to a local variable, and pass it as a parameter to
sh73a0_cpg_register_clock().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/5423e43d0cf518691965412fb510097d23ac5955.1654694831.git.geert+renesas@glider.be