powerpc/64: Allocate pacas per node

Per-node allocations are possible on 64s with radix that does
not have the bolted SLB limitation.

Hash would be able to do the same if all CPUs had the bottom of
their node-local memory bolted as well. This is left as an
exercise for the reader.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add dummy definition of boot_cpuid for !SMP]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>