mm: make memblock_alloc_base_nid() non-static

This will be used by powerpc to allocate per-cpu stacks and other
data structures node-local where possible.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Drop stray change to memblock_alloc_range() as noticed by akpm]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>