ARM: dts: rmobile: Move the u-boot,dm-pre-reloc into u-boot DTS on porter

Fix ommission where the u-boot,dm-pre-reloc DT bit was pulled into the
common DT, not the U-Boot specific DT part. Move it to U-Boot DT part.

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