powerpc: e500mc: Fix: use CONFIG_PPC_E500MC in idle_e500.S

It is wrongly using undefined CONFIG_E500MC.
Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/idle_e500.S

@@ -26,7 +26,7 @@ _GLOBAL(e500_idle)
 	ori	r4,r4,_TLF_NAPPING	/* so when we take an exception */
 	stw	r4,TI_LOCAL_FLAGS(r3)	/* it will return to our caller */
 
-#ifdef CONFIG_E500MC
+#ifdef CONFIG_PPC_E500MC
 	wrteei	1
 1:	wait