PEL: Maintenance procedure callout definitions

A maintenance procedure is a short string that resides in the FRU
identity structure in the SRC section of a PEL that maps to a service
procedure in the service documentation.

This commit adds the initial framework for dealing with these by adding
an enum type to specify them, and a pel_values.hpp function to get the
actual string value of the maintenance procedure based on its enum.

A single procedure is created in this commit,  NoVPDforFRU, which will
be used in a future commit when the location code, part number, etc,
aren't available when trying to do a callout and so this procedure will
be used instead.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I45f366e98794462c70d851f743e4f497c0af77b4
diff --git a/extensions/openpower-pels/registry/schema/registry_example.json b/extensions/openpower-pels/registry/schema/registry_example.json
index 89ae168..84071ec 100644
--- a/extensions/openpower-pels/registry/schema/registry_example.json
+++ b/extensions/openpower-pels/registry/schema/registry_example.json
@@ -63,7 +63,7 @@
                                 [
                                     {
                                         "Priority": "high",
-                                        "Procedure": "PSUPPLY"
+                                        "Procedure": "no_vpd_for_fru"
                                     }
                                 ]
                             }