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/schema.json b/extensions/openpower-pels/registry/schema/schema.json
index 035e538..1921ebd 100644
--- a/extensions/openpower-pels/registry/schema/schema.json
+++ b/extensions/openpower-pels/registry/schema/schema.json
@@ -394,9 +394,9 @@
 
         "procedure":
         {
-            "description": "The 7 character procedure callout name.",
+            "description": "The maintenance procedure callout.",
             "type": "string",
-            "pattern": "^[A-Z0-9]{7}$"
+            "enum": ["no_vpd_for_fru"]
         },
 
         "calloutType":