schemas: legacy: extract I2CMux

This allows to remove some properties from 'Aggregate' schema which are
no longer needed, since there is now a more specific schema to use.

Tested: Schema Validator Pass

Change-Id: I69ad69475a03bee6f365fc8c7fbd063bf200121f
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/schemas/global.json b/schemas/global.json
index 1985347..c03f32f 100644
--- a/schemas/global.json
+++ b/schemas/global.json
@@ -70,6 +70,9 @@
                             "$ref": "legacy.json#/$defs/IpmbSensor"
                         },
                         {
+                            "$ref": "legacy.json#/$defs/I2CMux"
+                        },
+                        {
                             "$ref": "virtual_sensor.json#/$defs/VirtualSensor"
                         },
                         {