ARM: vexpress: remove redundant vexpress_dt_cpus_num to get cpu count

arm_dt_init_cpu_maps parses the device tree, validates and sets the
cpu_possible_mask appropriately. It is unnecessary to do another DT
parse to get the number of cpus, use num_possible_cpus instead.

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

Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>