HPIO config settings needed for litedram

[daveshah1]

Being able to build litedram through this would be a medium-term target for me, as I already had it working with the nextpnr+RapidWright flow (subject to bitrot, it may still even be working).

This will require the following extra HPIO modes/options to be fuzzed:

• DIFF_POD12_DCI input/output (DQS), DIFF_SSTL12_DCI output (DRAM clock)
• PRE_EMPHASIS
• EQUALIZATION
• INTERNAL_VREF

Reference: https://github.com/litex-hub/litex-boards/blob/master/litex_boards/platforms/zcu104.py and https://github.com/litex-hub/litex-boards/blob/master/litex_boards/platforms/mercury_xu5.py