staging: vchiq: Move conditional barrier definition into vchiq_core.h

The barrier is only used by core code. So keep the barrier definition in
the core header.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-45-nsaenzjulienne@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>