spd: opteed: enable NS_TIMER_SWITCH

Enable dispatcher to save/restore unbanked timer registers. So that
both secure (OP-TEE) and non-secure (Linux) worlds can have independent
access control over timer registers.
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I4d58d5ff8298587ed478c8433fcbc3aef538d668

services/spd/opteed/opteed.mk

 #
-# Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -13,3 +13,6 @@ SPD_SOURCES		:=	services/spd/opteed/opteed_common.c	\
 				services/spd/opteed/opteed_pm.c
 
 NEED_BL32		:=	yes
+
+# required so that optee code can control access to the timer registers
+NS_TIMER_SWITCH		:=	1