clk: renesas: rcar-gen3: Add RPC clocks

The RPCSRC internal clock is controlled by the RPCCKCR.DIV[4:3] on all
the R-Car gen3 SoCs except V3M (R8A77970) but the encoding of this field
is different between SoCs; it makes sense to support the most common case
of this encoding in the R-Car gen3 CPG driver...

After adding the RPCSRC clock, we can add the RPC[D2] clocks derived from
it and controlled by the RPCCKCR register on all the R-Car gen3 SoCs except
V3M (R8A77970); the composite clock driver seems handy for this task, using
the spinlock added in the previous patch...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>