clk: renesas: cpg-lib: Move RPC clock registration to the library

We want to reuse this code for V3U soon. Because its RPCCKCR register is
at a different offset, the moved functions do not use the base register
as an argument anymore but the RPCCKCR register itself. Verified that an
Eagle board with R-Car V3M still works.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20211006085836.42155-2-wsa+renesas@sang-engineering.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>