clk: renesas: rza1: Remove struct rz_cpg

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

After this, the data member is the only remaining member of the rz_cpg
structure, so the whole structure can be replaced by the data member.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/2380285576edaa4ad3dc5eca7e0ca418f068c6ef.1654694831.git.geert+renesas@glider.be