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>

arch/powerpc/mm/pgtable_64.c

@@ -68,7 +68,7 @@
 unsigned long ioremap_bot = IOREMAP_BASE;
 
 #ifdef CONFIG_PPC_MMU_NOHASH
-static void *early_alloc_pgtable(unsigned long size)
+static __ref void *early_alloc_pgtable(unsigned long size)
 {
 	void *pt;