arm64: dts: renesas: Add Renesas R8A77990 SoC support

This patch adds basic support for the Renesas R-Car E3 (R8A77990) SoC:
  - PSCI
  - CPU (single)
  - Cache controller
  - Main clocks and controller
  - Interrupt controller
  - Timer
  - PMU
  - Reset controller
  - Product register
  - System controller
  - UART for console

Inspried by a patch by Takeshi Kihara in the BSP.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>