powerpc/85xx: Fix setting of EPAPR_MAGIC value

Had a typo in the ifdef for 85xx, should be CONFIG_MPC85xx for it to get
triggered.  Was pull in the non-BookE magic number.

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

arch/powerpc/lib/bootm.c

@@ -87,7 +87,7 @@ static void boot_jump_linux(bootm_headers_t *images)
 		 *   r8: 0
 		 *   r9: 0
 		 */
-#if defined(CONFIG_85xx) || defined(CONFIG_440)
+#if defined(CONFIG_MPC85xx) || defined(CONFIG_440)
  #define EPAPR_MAGIC	(0x45504150)
 #else
  #define EPAPR_MAGIC	(0x65504150)