ACPI / NUMA: add a stub function for node_to_pxm()

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

include/acpi/acpi_numa.h

@@ -30,6 +30,10 @@ static inline int pxm_to_node(int pxm)
 {
 	return 0;
 }
+static inline int node_to_pxm(int node)
+{
+	return 0;
+}
 #endif				/* CONFIG_ACPI_NUMA */
 
 #ifdef CONFIG_ACPI_HMAT