dts_fixups: Update labels generated from DTS with DT_ prefix

All labels containing "_<8-hex-digits>_" or "16550_<3or6-hex-digits>_"
in their names, assumed to be generated by the extracting script,
are updated with the DT_ prefix, to reflect the recent changes made
to the script.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>