There is already a stub function for pxm_to_node but conversion to the
other direction is missing.

It will be used by Microsoft Hypervisor code later.
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20210203150435.27941-9-wei.liu@kernel.org