doc: FIT image: fix incorrect description of DT node unit address

doc: FIT image: fix incorrect description 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.
Fix all occurences in the FIT image documentation files where this was not
observed, to not give bad examples to the reader.

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