clk: renesas: r8a7740: Remove r8a7740_cpg.reg

The register block base pointer as stored in the reg member of the
r8a7740_cpg structure is only used during initialization.  Hence move
it to a local variable, and pass it as a parameter to
r8a7740_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/7ec676bcc36ef1eda02c2db328c527fc5fd44e99.1654694831.git.geert+renesas@glider.be