Documentation: dt-bindings: update xhci-platform DT binding

This commit extends the compatible string list of the xhci-platform
binding with the new "armada-375-xhci" and "armada-380-xhci"
compatible strings. It is used to describe the XHCI controller which
is available in the Armada 375 and 38x SoCs.

It also indicates that an optional 'clocks' property is now supported.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

 USB xHCI controllers
 
 Required properties:
-  - compatible: should be "generic-xhci" (deprecated: "xhci-platform").
+  - compatible: should be one of "generic-xhci",
+    "marvell,armada-375-xhci", "marvell,armada-380-xhci" (deprecated:
+    "xhci-platform").
   - reg: should contain address and length of the standard XHCI
     register set for the device.
   - interrupts: one XHCI interrupt should be described here.
 
+Optional property:
+  - clocks: reference to a clock
+
 Example:
 	usb@f0931000 {
 		compatible = "generic-xhci";