arm64: introduce CPU_OF_TABLES for cpu ops selection

Add support to arm64 to provide a dt-based method to
allow soc-vendors to supply cpu_ops. Also move psci and
smp_spin_table ops to use CPU_OF_TABLES.

Change-Id: I4fd5b330fa3a7436fb88fff385deeb80a90548d6
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>