tools: fix incorrect usage of DT node unit address

tools: fix incorrect usage of DT node unit address

The DT spec demands a unit-address in a node name to match the "reg"
property in that node. Newer dtc versions will throw warnings if this is
not the case.
Correct the generated unit names when U-Boot's mkimage creates a FIT
image.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>