ACPI / OSL: Add kerneldoc comments to memory mapping functions

Add kerneldoc comments to acpi_os_map_iomem() and acpi_os_unmap_iomem()
and explain why the latter needs the __ref annotation in one of them
(as suggested by Mathias Krause).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mathias Krause <minipli@googlemail.com>