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 <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250220164655.2081224-1-Frank.Li@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>