arm64: dts: renesas: r8a77990: Use CPG/MSSR and SYSC binding definitions

Use the SoC-specific CPG/MSSR include file to allow future use of
R8A77990_CLK_* symbols.
Replace the hardcoded power domain indices by R8A77990_PD_* symbols.

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