arm64: dts: r8a7796: Add Renesas R8A7796 SoC support

Basic support for the Gen 3 R-Car M3-W SoC.

Based on work for the r8a7795 and r8a7796 SoCs by
Takeshi Kihara, Dirk Behme and Geert Uytterhoeven.

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