PMIC 5VSYS pin is a power input (not output)

[oskirby]

The MxL7704 datasheet leaves the purpose of the 5VSYS pin a bit ambiguous as to whether it is internally connected to VIN, or whether it's requires an external RC filter to VIN. Turns out it needs an external RC filter. We can work around it on the prototypes by bodging a 10ohm resistor between 5VSYS and VBUS, but this should be fixed for future production runs.