Merge changes from topic "bk/context_refactor" into integration

* changes:
  refactor(psci): extract cm_prepare_el3_exit_ns() to a common location
  refactor(cm): set MDCR_EL3/CPTR_EL3 bits in respective feat_init_el3() only
  fix(cm): set MDCR_EL3.{NSPBE, STE} explicitly
  refactor(cm): factor out EL2 register setting when EL2 is unused