PCI: mvebu: Handle changes to the bridge windows while enabled

The PCI core will write to the bridge window config multiple times while
they are enabled.  This can lead to mbus failures like this:

 mvebu_mbus: cannot add window '4:e8', conflicts with another window
 mvebu-pcie mbus:pex@e0000000: Could not create MBus window at [mem 0xe0000000-0xe00fffff]: -22

For me this is happening during a hotplug cycle.  The PCI core is not
changing the values, just writing them twice while active.

The patch addresses the general case of any change to an active window, but
not atomically.  The code is slightly refactored so io and mem can share
more of the window logic.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>