ARM: dts: rmobile: Import R8A7792 DTS from Linux 4.15-rc8

Import the Renesas R8A7792 DTS and headers from upstream Linux kernel v4.15-rc8,
commit bebc6082da0a9f5d47a1ea2edc099bf671058bd4 .

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>