xen/arm: mm: Use PAGE_HYPERVISOR_* instead of MT_* when calling set_fixmap

xen/arm: mm: Use PAGE_HYPERVISOR_* instead of MT_* when calling set_fixmap

At the moment, PAGE_HYPERVISOR_* and MT_* have exactly the same value.
In a follow-up patch the former will be extended to carry more
information.

It looks like the caller of set_fixmap are mixing the both. Stay
consistent and only use PAGE_HYPERVISOR_*. This is also match the
behavior of create_xen_entries and would potentially allow to share some
part in the future.

Also rename the parameter 'attributes' to 'flags' so it is clearer what
is the interface.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>