powerpc/64e: Add __ref to early_alloc_pgtable()

This silences a section mismatch warning.  early_alloc_pgtable() is
called from map_kernel_page() which cannot be __init, but only when
slab_is_available() returns false which can only happen during early
boot.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>