I've been working with FT2232H a lot but I don't use FTDI drivers, instead I work directly through libusb.
1. Does FT2232H support 30MHz at JTAG-TCK?
IIRC, yes, I think I measured it at some point.
2. Is it required for platform to provide 60MHz clock to FTDI or 12MHz crystal is sufficient? Or any other hardware change required?
12 MHz crystal is sufficient, nothing else is needed in HW.
3. Any reference driver/test to enable FT2232H JTAG TCK operating at 30MHz?
These are notes from my code:
Master clock is 60 MHz after /5 clock divider is disabled
required_clock = master_clock / (2 * (divider + 1))
divider = master_clock / required_clock / 2 - 1
If you set it to 30 MHz but actually measure 6 MHz then the /5 clock divider is enabled.