xen/arm: p2m: Fix incorrect mapping of superpages

xen/arm: p2m: Fix incorrect mapping of superpages

The same set of functions is used to set as well as to clean P2M
entries, except for clean operations (INVALID_MFN ~0UL) is passed as a
parameter. Unfortunately, when calculating an appropriate target order
for a particular mapping INVALID_MFN is taken into account which leads
to 4K page target order being set each time even for 2MB and 1GB
mappings.

This will result to break down the superpage into 4K mappings and leave
empty tables allocated.

This was introduced by commit 2ef3e36ec7 "xen/arm: p2m: Introduce
p2m_set_entry and __p2m_set_entry".

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