usb: typec: tcpci: Implement callbacks for FRS

Implement tcpc.enable_frs to enable TCPC to receive
Fast role swap signal.

Additionally set the sink disconnect threshold to 4v
to prevent disconnect during Fast Role swap.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20201008061556.1402293-8-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>