ARM: zynq: remove unnecessary setting of cpu_present_mask

This patch also removes setting cpu_present_mask as platforms should
only re-initialize it in smp_prepare_cpus() if present != possible.

Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>