dt-bindings: mmc: Change to additionalProperties to fix fail detect Unevaluated property

mmc-controller.yaml is common schema file. According to writing-schema.rst,

* additionalProperties: true
   Rare case, used for schemas implementing common set of properties. Such
   schemas are supposed to be referenced by other schemas, which then use
   'unevaluatedProperties: false'.  Typically bus or common-part schemas.

Reproduce steps:
1. Add unevaluated property 'abc' at example of fsl-imx-esdhc.yaml
2. Run make dt_binding_check DT_SCHEMA_FILES=fsl-imx-esdhc.yaml

No any warning report. But suppose report below warning:
mmc@70004000: Unevaluated properties are not allowed ('abc' was unexpected)

Signed-off-by: Frank Li <[email protected]>
Reviewed-by: Rob Herring (Arm) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ulf Hansson <[email protected]>

Author: nxpfrankli

Documentation/devicetree/bindings/mmc/mmc-controller.yaml

@@ -24,7 +24,7 @@ properties:
   $nodename:
     pattern: "^mmc(@.*)?$"
 
-unevaluatedProperties: true
+additionalProperties: true
 
 examples:
   - |