arm: common: Make timer configuration conditional

Make the timer configuration conditional on the optional interface being
available.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

plat/arm/common/aarch64/arm_common.c

@@ -137,6 +137,7 @@ uint32_t arm_get_spsr_for_bl33_entry(void)
 /*******************************************************************************
  * Configures access to the system counter timer module.
  ******************************************************************************/
+#ifdef ARM_SYS_TIMCTL_BASE
 void arm_configure_sys_timer(void)
 {
 	unsigned int reg_val;
@@ -151,6 +152,7 @@ void arm_configure_sys_timer(void)
 	reg_val = (1 << CNTNSAR_NS_SHIFT(PLAT_ARM_NSTIMER_FRAME_ID));
 	mmio_write_32(ARM_SYS_TIMCTL_BASE + CNTNSAR, reg_val);
 }
+#endif /* ARM_SYS_TIMCTL_BASE */
 
 /*******************************************************************************
  * Returns ARM platform specific memory map regions.