ARM: dts: rmobile: Add rudimentary R8A7790 Stout DT

The Stout board is not supported in Linux, so add a rudimentary DTS
for H2 Stout as a placeholder for when a proper DTS is available.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
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NOTE: The Stout is already being shipped over, so proper DTS for both
      U-Boot and Linux will happen once it's delivered :-)