ARM: Define PCI_IOBASE as the base of virtual PCI IO space

This is needed for calls into OF code that parses PCI ranges.  It signals
support for memory mapped PCI I/O accesses that are described by device
trees.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
CC: Russell King <linux@arm.linux.org.uk>
CC: Rob Herring <robh+dt@kernel.org>