powerpc: mmu_radix_page_array_startup(): Don't force allocation from domain 0

In this case, the domain from which the memory to back the VM page array
is allocated does not matter, so just let vm_phys_early_alloc() choose
a suitable domain.

Reviewed by: jhibbits, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48888