arm: dts: r8a774b1: Import DTS from Linux 5.9-rc4

Import R8A774B1 (RZ/G2N) SoC DTSI and headers from upstream Linux kernel
5.9-rc4 commit f4d51dffc6c0 ("Linux 5.9-rc4")

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>