arm64: dts: renesas: r8a78000: Add CPG/MDLC nodes

Add device nodes for the Clock Pulse Generator (CPG) and Module Control
(MDLC) blocks on the R-Car X5H (R8A78000) SoC.

Convert all (H)SCIF serial ports from dummy to CPG clocks, and link them
to an MDLC for power domains and resets.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
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Add all MDLC nodes from the start, or only when used/tested?