usb: host: xhci-plat: Fix clock resource by adding a register clock

On Armada 7K/8K we need to explicitly enable the register clock. This
clock is optional because not all the SoCs using this IP need it but at
least for Armada 7K/8K it is actually mandatory.

The change was done at xhci-plat level and not at a xhci-mvebu.c because,
it is expected that other SoC would have this kind of constraint.

The binding documentation is updating accordingly.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -28,7 +28,10 @@ Required properties:
   - interrupts: one XHCI interrupt should be described here.
 
 Optional properties:
-  - clocks: reference to a clock
+  - clocks: reference to the clocks
+  - clock-names: mandatory if there is a second clock, in this case
+    the name must be "core" for the first clock and "reg" for the
+    second one
   - usb2-lpm-disable: indicate if we don't want to enable USB2 HW LPM
   - usb3-lpm-capable: determines if platform is USB3 LPM capable
   - quirk-broken-port-ped: set if the controller has broken port disable mechanism