arm: zynq: Don't use arm_global_timer with cpufreq

The timer frequency of the arm_global_timer depends on the CPU
frequency. With cpufreq altering that frequency the arm_global_timer
does not maintain a stable time base. Therefore don't enable that timer
in case cpufreq is enabled.

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