arm64: dts: renesas: Initial r8a774b1 SoC device tree

Basic support for the RZ/G2N (R8A774B1) SoC. Added placeholders
to avoid compilation error with the common platform code.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1569589584-56917-1-git-send-email-biju.das@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>