dt-bindings: serial: correct ref to serial.yaml

dt-bindings: serial: correct ref to serial.yaml

The serial bindings should reference the local (in kernel) serial.yaml,
not the /schemas/serial.yaml.  The latter comes from dtschema package
and is a small subset of serial bindings.

Usage of the local serial.yaml allows typical properties and children:

  xilinx/avnet-ultra96-rev1.dtb: serial@ff000000: Unevaluated properties are not allowed ('bluetooth' were unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Michal Simek <michal.simek@amd.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230124091916.45054-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>