dt-bindings: serial: 8250: Remove duplicated compatible strings

The compatible strings "mediatek,*" appears two times, remove one of them.

Fixes: e69f5dc6

 ("dt-bindings: serial: Convert 8250 to json-schema")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210422090857.583-1-thunder.leizhen@huawei.com
Signed-off-by: Rob Herring <robh@kernel.org>