ARM: dts: rmobile: Extract i2c6 on M2W Porter

The i2c6 node is missing in mainline Linux thus far, pull it
into U-Boot specific DT until it hits mainline Linux, to make
syncing of DTs easier.

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