Modify acpi_os_map_generic_address() to return the pointer returned
by acpi_os_map_iomem() which represents the logical address
corresponding to the struct acpi_generic_address argument passed to
it or NULL if that address cannot be obtained (for example, the
argument does not represent an address in system memory or it could
not be mapped by the OS).

Among other things, that will allow the ACPI OS layer to pass the
logical addresses of the FADT GPE blocks 0 and 1 to ACPICA going
forward.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>