bus: mvebu-mbus: Export symbols for public API window functions

This would allow to compile pci-mvebu.c driver as module.

Link: https://lore.kernel.org/r/20211126144307.7568-2-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>