xen/arm: Don't build GICv3 with the new vGIC

xen/arm: Don't build GICv3 with the new vGIC

Commit 54ec59f6b0 "xen/arm: vgic-v3: Don't create empty re-distributor
regions" breaks compilation when using the new vGIC.

This is because the field nr_regions is not existing in the vgic
structure. For simplicity, as vGICv3 is not yet imported, disable GICv3.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 62aa9e7f1b8ef64b8c7c1dacb1122351cb9fd132)