xen/arm: fix booting ACPI based system after static evtchn series

xen/arm: fix booting ACPI based system after static evtchn series

When ACPI is enabled and the system booted with ACPI, BUG() is observed
after merging the static event channel series. As there is no DT when
booted with ACPI there will be no chosen node because of that
"BUG_ON(chosen == NULL)" will be hit.

(XEN) Xen BUG at arch/arm/domain_build.c:3578

Move call to alloc_static_evtchn() under acpi_disabled check to fix the
issue.

Fixes: 1fe16b3ed78a (xen/arm: introduce xen-evtchn dom0less property)
Signed-off-by: Rahul Singh <rahul.singh@arm.com>
[stefano: minor spelling fix in commit message]
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Ayan Kumar Halder <ayankuma@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>