xen/arm: gic: Make it clear the GIC node is passed to make_hwdom_dt_node

xen/arm: gic: Make it clear the GIC node is passed to make_hwdom_dt_node

The callback make_hwdom_dt_node already has the GIC node in parameter.

Rather than using a weird mix between "dt_interrupt_controller" (aliased
to "gic") and "node", rename the callback parameter "node" to "gic" and
remove local GIC definitions in terms of the global
dt_interrupt_interrupt_controller.

Also, add an assert to gic_make_hwdom_dt_node to check that the GIC
really is the global dt_interrupt_controller.

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