arm: mvebu: armada-xp-theadorable.dts: Move u-boot, dm-pre-reloc to -u-boot.dtsi

Move U-Boot specific device tree property u-boot,dm-pre-reloc into U-Boot
specific device tree include file armada-xp-theadorable-u-boot.dtsi.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>