powerpc: Use the page allocator

The spapr_hcall test makes two page sized allocations using the physical
allocator. Initialize the page allocator and use alloc_page() directly.

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>