ACPI: Rename acpi_map_pxm_to_online_node() to pxm_to_online_node()

As this function is no longer allowed to create new mappings
let us rename it to reflect this.

Note all nodes should already exist before any of the users
of this function are called.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>