dt-bindings: serial: add the required property 'additionalProperties'

dt-bindings: serial: add the required property 'additionalProperties'

When I do dt_binding_check for any YAML file, below wanring is always
reported:

xxx/serial/litex,liteuart.yaml: 'additionalProperties' is a required property
xxx/serial/litex,liteuart.yaml: ignoring, error in schema:
warning: no schema found in file: xxx/serial/litex,liteuart.yaml

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201204093813.1275-3-thunder.leizhen@huawei.com
Signed-off-by: Rob Herring <robh@kernel.org>