xen/arm: gic: Refactor the code for creating gic node

xen/arm: gic: Refactor the code for creating gic node

Since fdt_begin_node() is called by all gicXX_make_dt_node() to create
the interrupt-controller devicetree node, this patch refactors the call
and moves it inside make_gic_node(). This also matches the fdt_end_node()
call at the end of make_gic_node().

This patch also move the call to gic_make_node() wrapper to be after all
other generic properties are setup. This allows creating sub-node inside
gic_make_node() in order to support v2m sub-node.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>