docs/hw/falcon/io: Add missing Falcon registers UNK174[] & UNK17C
docs/hw/falcon/io: Add missing Falcon registers UNK174[] & UNK17C

These MMIO registers were noted in rnndb/falcon.xml, although presently lack
full documentation.

UNK174[] is believed to have a length of 2 (each 0x4).

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