arm: Update cpu_possible_mask according to CPU definition in DT

Since the boot CPU node contains proper data, and all nodes have
a reg property, the DT CPU list can be considered valid and the
cpu_possible_mask initialized in msm_smp_init_cpus() can be
overriden. What's more, the remaining value of default mask
should be cleared in case that the non-existent cores are
included by accident.

Change-Id: I033c42e1f235ac68ea59c7fb27b0067a6c80e16f
Signed-off-by: Xiaocheng Li <lix@codeaurora.org>