arm: use new phandle allocation functions

To refer to the GIC FDT node, we used to pass the GIC phandle to most
of the functions dealing with FDT nodes.
Since we now have a global phandle reference, use that to refer to the
GIC handle in various places and get rid of the now unneeded parameter
passing.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>