ARM: Allow unmapping of fixed PCI I/O mappings

ARM: Allow unmapping of fixed PCI I/O mappings

The new pci_iounmap_io() function can be used to unmap a fixed PCI I/O
mapping as established by pci_ioremap_io(). This will be useful to
support unbinding of PCI host drivers.

vidyas: make similar change for ARM64
based on http://git-master/r/#/c/450679

Bug 1496843

Change-Id: I5d1cbcf4f66b89b9711d6897ed444b680d20d613
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Reviewed-on: http://git-master/r/454937
GVS: Gerrit_Virtual_Submit
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>