schemas: legacy: extract GenericSMBusMux

Unknown use-case.

Create a schema anyways for completeness sake.

Tested: Schema Validator Pass

Change-Id: I95bc4efe8feb950a66cd8e0732fe4b76faed6214
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/schemas/global.json b/schemas/global.json
index 04213d9..337f7c0 100644
--- a/schemas/global.json
+++ b/schemas/global.json
@@ -133,6 +133,9 @@
                             "$ref": "legacy.json#/$defs/IntelHsbpCPLD"
                         },
                         {
+                            "$ref": "legacy.json#/$defs/GenericSMBusMux"
+                        },
+                        {
                             "$ref": "virtual_sensor.json#/$defs/VirtualSensor"
                         },
                         {