arm64: dts: renesas: Add Gray Hawk Single board support

arm64: dts: renesas: Add Gray Hawk Single board support

Add initial support for the Renesas Gray Hawk Single board, which is
based on the R-Car V4M (R8A779H0) SoC:
  - Memory,
  - Crystal oscillators,
  - Serial console.

Based on the White Hawk Single DTS, and on a patch for the Gray Hawk
board stack in the BSP by Hai Pham.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/b657402113267acd57aece0b4c681b707e704455.1706194617.git.geert+renesas@glider.be