Enable/test PPS with various sinks

[manuelkasper]

Currently PPS is disabled in the firmware, even though the hardware would basically support it. Before announcing PPS capability to PD sinks, ensure that the timing between reaching the requested voltage and issuing PS_RDY is OK.

[manuelkasper]

Apparently the FUSB302's MDAC is not accurate enough for PPS (see comment in isVBUSOverVoltage()), so we'd need to use the charger IC's ADC.