ARM: dts: rmobile: Extract RPC node to u-boot specific DT

ARM: dts: rmobile: Extract RPC node to u-boot specific DT

The RPC DT bindings are still work in progress. Extract the RPC DT node
from the DT to allow easier update and so it can be replaced once the DT
bindings are stable.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
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V2: Rebase on u-boot/master