clk: renesas: rcar-gen3: Factor out CPG library

clk: renesas: rcar-gen3: Factor out CPG library

R-Car V3U has a CPG different enough to not be a generic Gen3 CPG but
similar enough to reuse code. Introduce a new CPG library, factor out
the SD clock handling and hook it to the generic Gen3 CPG driver so we
have an equal state. V3U will make use of it in the next patch then.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20201227174202.40834-4-wsa+renesas@sang-engineering.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>