arm: mvebu: armada-370-xp.dtsi: Add "u-boot, dm-pre-reloc" to "internal-regs"

Without this U-Boot specific property, booting on Armada XP theadorable
fails in SPL. All nodes in the "internal-regs" (simple-bus) DT node are
not scanned, so the UART node is missing (and others).

I'm not adding this property in an *u-boot.dtsi file, since there is
none matching the generic rules for all files including this dtsi
file. So to not miss any of the boards using this dtsi file, I'm
adding it to this file directly, which makes the Linux merge a less
easy unforunately.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Chris Packham <judge.packham@gmail.com>
Cc: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Chris Packham <judge.packham@gmail.com>