clk: renesas: cpg-mssr: Extract common R-Car Gen3 support code

Extract the code to support parts common to all members of the R-Car
Gen3 SoC family into a separate file, to ease sharing among SoC-specific
drivers.

Note that while the cpg_pll_configs[] arrays and the selection of the
config based on the MODE bits are identical on R-Car H3 and R-Car M3-W,
they are not common, and may be different on other R-Car Gen3 SoCs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Simon Horman <horms+renesas@verge.net.au>