clk: renesas: r8a774c0: Add RPC clocks

Describe the RPCSRC internal clock and the RPC[D2] clocks derived from it,
as well as the RPC-IF module clock, in the RZ/G2E (R8A774C0) CPG/MSSR
driver.

Add new clk type CLK_TYPE_GEN3_E3_RPCSRC to register rpcsrc as a fixed
clock on R-Car Gen3 E3 (and also RZ/G2E which is identical to E3 SoC),
parent and the divider is set based on the register value CPG_RPCCKCR[4:3]
which has been set prior to booting the kernel.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20201116101002.5986-1-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>