The latest RPi firmware leaves secondary cores in a wait-for-event (WFE)

state to save power, so after writing the entry point address for a core to
the mailbox, use a dsb() to synchronize the execution pipeline to the data
written, then use an sev() to wake up the core.

Submitted by:	Sylvain Garrigues <sylgar@gmail.com>

sys/arm/broadcom/bcm2835/bcm2836_mp.c

@@ -117,7 +117,9 @@ bcm2836_mp_start_ap(platform_t plat)
 		/* set entry point to mailbox 3 */
 		BSWR4(MBOX3SET_CORE(i),
 		    (uint32_t)pmap_kextract((vm_offset_t)mpentry));
-		wmb();
+		/* Firmware put cores in WFE state, need SEV to wake up. */
+		dsb();
+        sev();
 
 		/* wait for bootup */
 		retry = 1000;