xen/arm: dom0less: cope with missing /gic phandle

xen/arm: dom0less: cope with missing /gic phandle

If a partial DT has a /gic node, but no references to it, dtc may omit
the phandle property. With the phandle property missing,
fdt_get_phandle() returns 0, leading Xen to generate a malformed domU
dtb due to invalid interrupt-parent phandle references. 0 is an invalid
phandle value. Add a zero check, and fall back to GUEST_PHANDLE_GIC.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>