powerpc/qman: Change fsl,qman-channel-id to cell-index

It turns out that existing U-Boots will dereference NULL pointers
if the device tree does not have cell-index in the portal nodes.

No patch has yet been merged adding device tree nodes for this binding
(except a dtsi that has not yet been referenced), nor has any driver
yet been merged making use of the binding, so it's not too late to
change the binding in order to keep compatibility with existing
U-Boots.
Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Madalin-Cristian Bucur <madalin.bucur@freescale.com>

Documentation/devicetree/bindings/soc/fsl/qman-portals.txt

@@ -47,7 +47,7 @@ PROPERTIES
 
 	For additional details about the PAMU/LIODN binding(s) see pamu.txt
 
-- fsl,qman-channel-id
+- cell-index
 	Usage:		Required
 	Value type:	<u32>
 	Definition:	The hardware index of the channel. This can also be
@@ -136,7 +136,7 @@ The example below shows a (P4080) QMan portals container/bus node with two porta
 			reg = <0x4000 0x4000>, <0x101000 0x1000>;
 			interrupts = <106 2 0 0>;
 			fsl,liodn = <3 4>;
-			fsl,qman-channel-id = <1>;
+			cell-index = <1>;
 
 			fman0 {
 				fsl,liodn = <0x22>;