xen: arm: correct name of the dt node passed around when building dom0 DT

xen: arm: correct name of the dt node passed around when building dom0 DT

In the case of the GIC, timer and write_properties the argument is the node,
not the parent. Rename the argument to "node" in this case.

I think this stems from the use of the name "np" in other places (confusing
"node parent" for "node pointer").  Therefore replace all uses of "np" with
"node".

In addition in write_properties now that np=>node the name pp makes no sense.
Rename it to "prop".

No semantic change.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>