usb: typec: tcpm: Implement enabling Auto Discharge disconnect support

TCPCI spec allows TCPC hardware to autonomously discharge the vbus
capacitance upon disconnect. The expectation is that the TCPM enables
AutoDischargeDisconnect while entering SNK/SRC_ATTACHED states. Hardware
then automously discharges vbus when the vbus falls below a certain
threshold i.e. VBUS_SINK_DISCONNECT_THRESHOLD.

Apart from enabling the vbus discharge circuit, AutoDischargeDisconnect
is also used a flag to move TCPCI based TCPC implementations into
Attached.Snk/Attached.Src state as mentioned in
Figure 4-15. TCPC State Diagram before a Connection of the
USB Type-C Port Controller Interface Specification.
In such TCPC implementations, setting AutoDischargeDisconnect would
prevent TCPC into entering "Connection_Invalid" state as well.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20201029063138.1429760-8-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>