bus: mvebu-mbus: Add new API for the PCIe memory and IO aperture

We add two optional properties to the MBus DT binding, to encode
the PCIe memory and IO aperture. This allows such information to
be retrieved by -for instance- the pci driver to allocate the
MBus decoding windows.

Correspondingly, and in order to retrieve this information,
we add two new APIs.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>