armv8: secure firmware: fix incorrect unit address in node name

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.
Remove the unit address from the config node name when U-Boot deals with
secure firmware FIT images.

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