fix(zynqmp): ensure memory write finish with dsb()

GICD reg write must complete before core goes to idle
mode. Achieve this with dsb() barrier instruction in IPI
ISR

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Change-Id: I5af42ca901567ee5e54a5434ebe3e673a92cb9be

plat/xilinx/zynqmp/pm_service/pm_svc_main.c

@@ -151,6 +151,8 @@ static uint64_t __unused __dead2 zynqmp_sgi7_irq(uint32_t id, uint32_t flags,
 				0xffffffff);
 	}
 
+	dsb();
+
 	spin_unlock(&inc_lock);
 
 	if (active_cores == 0) {