Fill in <linux/acpi_io.h> with acpi_os_ioremap (= AcpiOsMapMemory).

Define __acpi_iomem as an empty frotz to replace the Linux __iomem
attribute selectively and greppably.  AcpiOsMapMemory returns usable
kernel virtual addresses, so we don't need to use bus_space_mumble or
anything to dereference them.

(riastradh)