xen: arm: Support the other 4 PCI buses on Xgene

Currently we only establish specific mappings for pcie0, which is
used on the Mustang platform. However at least McDivitt uses pcie3.
So wire up all the others, based on whether the corresponding DT node
is marked as available.

This results in no change for Mustang.

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