dt-bindings: serial: Change to reference the kernel-defined serial.yaml

dt-bindings: serial: Change to reference the kernel-defined serial.yaml

The /schemas/serial.yaml from dt-schema only has the property $nodename,
whereas the kernel-defined /schemas/serial/serial.yaml contains more
useful properties, support for more complex application scenarios.

For example:
1) The property "current-speed" in fsl-lx2160a.dtsi
2) The subnode "bluetooth" in hi3660-hikey960.dts

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210506132049.1513-2-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>