arm: zynq: Enable arm_global_timer

Zynq is based on an ARM Cortex-A9 MPCore, which features the
arm_global_timer in its SCU. Therefore enable the timer for Zynq.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>