arm64: dts: renesas: Add Renesas R8A77961 SoC support

Add initial support for the Renesas R-Car M3-W+ (R8A77961) SoC.

This includes:
  - Cortex-A57 and Cortex-A53 CPU cores
    (incl. L2 caches and power state definitions),
  - Power Management Unit,
  - PSCI firmware,
  - Pin Function Controller,
  - Clock, Reset, System, and Interrupt Controllers,
  - SCIF2 serial console,
  - Product Register,
  - ARM Architectured Timer,
and various placeholders to allow to use salvator-xs.dtsi.

Based on r8a7796.dtsi.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-9-geert+renesas@glider.be