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>