dt-bindings: serial: Add label property in serial.yaml

dt-bindings: serial: Add label property in serial.yaml

When there is more than one serial port present, the property 'label'
allows a custom name to be used for briefly describe the usage or position
of each serial port.

Without this "label" property, many dtbs_check warnings similar to the
following are reported:
arch/arm64/boot/dts/hisilicon/hi3660-hikey960.dt.yaml: \
serial@ffd74000: Additional properties are not allowed ('label' was unexpected)
        From schema: Documentation/devicetree/bindings/serial/pl011.yaml

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210506132049.1513-3-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>