usb: pd: Fully support PD 3.0 as dual-role

As the PD PHY hardware now fully supports sending and receiving
of all PD 3.0 messages, add support for PD 3.0 source mode and
remove the sink-only limitation to fully support dual-role.

When operating in PD 3.0 source mode, the value of the Rp pullup
resistance must be adjusted between SinkTxOk (3A) and SinkTxNG
(1.5A) to indicate when a sink can initiate an atomic message
sequence (AMS) as a collision avoidance scheme. Use the new usb
power_supply property to change the source Rp value to implement
helper functions that can mark the start/end of an AMS at various
stages in the PE_SRC_READY state.

Change-Id: I2bec1d0cb5244fcb5a0fbf13985545d84e0f0b17
Signed-off-by: Jack Pham <jackp@codeaurora.org>