powerpc: Don't spin on sync instruction at boot time

Push the sync below the secondary smp init hold loop and comment its purpose.
This should speed up boot by reducing global traffic during the single-threaded
portion of boot.

Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_64.S

@@ -1198,7 +1198,6 @@ _GLOBAL(generic_secondary_smp_init)
 3:	HMT_LOW
 	lbz	r23,PACAPROCSTART(r13)	/* Test if this processor should */
 					/* start.			 */
-	sync
 
 #ifndef CONFIG_SMP
 	b	3b			/* Never go on non-SMP		 */
@@ -1206,6 +1205,8 @@ _GLOBAL(generic_secondary_smp_init)
 	cmpwi	0,r23,0
 	beq	3b			/* Loop until told to go	 */
 
+	sync				/* order paca.run and cur_cpu_spec */
+
 	/* See if we need to call a cpu state restore handler */
 	LOAD_REG_IMMEDIATE(r23, cur_cpu_spec)
 	ld	r23,0(r23)