Implement TPS2543 USB charging control.
It contains routine for setting each USB port as dedicated charging port
or standard downstream port. To allow us controlling the current
distributed to each port, we can select whether to allow 500mA or 1500mA
for each port.
BUG=chrome-os-partner:7476
TEST=Added USB port definition for BDS and tested GPIO output voltage
level is correct for all modes.
Change-Id: I19bc4b30d333aa802f868ebfc3a398b30e99ba0f