arm/p2m: Fix regression during domain shutdown with active mem_access

arm/p2m: Fix regression during domain shutdown with active mem_access

The change in commit 438c5fe4f0c introduced a regression for domains where
mem_acces is or was active. When relinquish_p2m_mapping attempts to clear
a page where the order is not 0 the following ASSERT is triggered:

    ASSERT(!p2m->mem_access_enabled || page_order == 0);

This regression was unfortunately not caught during testing in preparation
for the 4.8 release.

In this patch we adjust the ASSERT to not trip when the domain
is being shutdown.

Ideally this fix would be part of Xen 4.8.1.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Julien Grall <julien.grall@arm.com>