Merge changes from topic "panic_cleanup" into integration

* changes:
  refactor(bl31): use elx_panic for sysreg_handler64
  refactor(aarch64): rename do_panic and el3_panic
  refactor(aarch64): remove weak links to el3_panic
  refactor(aarch64): refactor usage of elx_panic
  refactor(aarch64): cleanup HANDLE_EA_EL3_FIRST_NS usage