[POWERPC] Fix swapper_pg_dir size when CONFIG_PTE_64BIT=y on FSL_BOOKE

The size of swapper_pg_dir is 8k instead of 4k when using 64-bit PTEs
(CONFIG_PTE_64BIT).

This was reported by Cedric Hombourger <chombourger@gmail.com>

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/head_44x.S

@@ -722,7 +722,7 @@ empty_zero_page:
  */
 	.globl	swapper_pg_dir
 swapper_pg_dir:
-	.space	8192
+	.space	PGD_TABLE_SIZE
 
 /* Reserved 4k for the critical exception stack & 4k for the machine
  * check stack per CPU for kernel mode exceptions */