dt-bindings: connector: Add self-powered property

>From USB_PD_R3_0
7.1.5 Response to Hard Resets
Device operation during and after a Hard Reset is defined as follows:
Self-powered devices Should Not disconnect from USB during a Hard Reset
(see Section 9.1.2).
Bus powered devices will disconnect from USB during a Hard Reset due to the
loss of their power source.

Therefore it is necessary to know whether the port belongs to
a device which is self powered or bus powered. This change
adds "self-powered" flag to the connector class which present indicates
that the port belongs to a device that is self powered. Else it is
bus powered usb device.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
-------
Changes is v3:
- Rebase on top of usb-next
- no change w.r.t to this patch. Same as previous versions.

No v2 version as the patch was introduced there.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/connector/usb-connector.txt

@@ -14,6 +14,8 @@ Optional properties:
 - label: symbolic name for the connector,
 - type: size of the connector, should be specified in case of USB-A, USB-B
   non-fullsize connectors: "mini", "micro".
+- self-powered: Set this property if the usb device that has its own power
+  source.
 
 Optional properties for usb-c-connector:
 - power-role: should be one of "source", "sink" or "dual"(DRP) if typec