schemas: legacy: enumerate 'SensorType' field

Tested: Schema Validator Pass

Change-Id: I45380cb8d3aac48d2406082a75e70d967d79be45
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/schemas/legacy.json b/schemas/legacy.json
index b251c8a..be634dc 100644
--- a/schemas/legacy.json
+++ b/schemas/legacy.json
@@ -860,7 +860,7 @@
                 "type": "number"
             },
             "SensorType": {
-                "type": "string"
+                "enum": ["current", "power", "voltage"]
             },
             "SlotId": {
                 "type": "string"