powerpc: Replace the physical allocator with the page allocator

The spapr_hcall test makes two page sized allocations using the physical
allocator. Replace the physical allocator with the page allocator, which
has has more features (like support for freeing allocations), and would
allow for further simplification of the physical allocator.

CC: Laurent Vivier <lvivier@redhat.com>
CC: Thomas Huth <thuth@redhat.com>
CC: kvm-ppc@vger.kernel.org
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>