This commit adds support for NRF52 TWI Master mode and slightly extends
existing I2C API. This is a breaking change, while mode selection needs to be
done during enabling I2C. There is one additional breaking change done because:
1) Unicore MX API design was PITA for writes
2) It is incompatible with EasyDMA
I strongly apologize to all two users who might be affected by this change.
