ARM: zynq: cpuidle: Remove pointless code

The core is not powered down, it is pointless to call the cpu_pm notifiers and
switch to the global timer.

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