xen/arm: Construct the P2M pages pool for guests

xen/arm: Construct the P2M pages pool for guests

This commit constructs the p2m pages pool for guests from the
data structure and helper perspective.

This is implemented by:

- Adding a `struct paging_domain` which contains a freelist, a
counter variable and a spinlock to `struct arch_domain` to
indicate the free p2m pages and the number of p2m total pages in
the p2m pages pool.

- Adding a helper `p2m_get_allocation` to get the p2m pool size.

- Adding a helper `p2m_set_allocation` to set the p2m pages pool
size. This helper should be called before allocating memory for
a guest.

- Adding a helper `p2m_teardown_allocation` to free the p2m pages
pool. This helper should be called during the xl domain destory.

This is part of CVE-2022-33747 / XSA-409.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>