PEL: peltool: Add const strings for section header

Added "Section Version", "Sub-section type" and "Created by" constant strings.

Updated all getJSON() functions to use the new constants.

Signed-off-by: Harisuddin Mohamed Isa <harisuddin@gmail.com>
Change-Id: I434192afed929972845c3cfb8876cc5ef784eec2
diff --git a/extensions/openpower-pels/src.cpp b/extensions/openpower-pels/src.cpp
index 858c243..8d2fd0b 100644
--- a/extensions/openpower-pels/src.cpp
+++ b/extensions/openpower-pels/src.cpp
@@ -431,11 +431,9 @@
 std::optional<std::string> SRC::getJSON(message::Registry& registry) const
 {
     std::string ps;
-    jsonInsert(ps, "Section Version", getNumberString("%d", _header.version),
-               1);
-    jsonInsert(ps, "Sub-section type", getNumberString("%d", _header.subType),
-               1);
-    jsonInsert(ps, "Created by", getNumberString("0x%X", _header.componentID),
+    jsonInsert(ps, pv::sectionVer, getNumberString("%d", _header.version), 1);
+    jsonInsert(ps, pv::subSection, getNumberString("%d", _header.subType), 1);
+    jsonInsert(ps, pv::createdBy, getNumberString("0x%X", _header.componentID),
                1);
     jsonInsert(ps, "SRC Version", getNumberString("0x%02X", _version), 1);
     jsonInsert(ps, "SRC Format", getNumberString("0x%02X", _hexData[0] & 0xFF),