dt-bindings: serial: add common properties schema for UART children

Properties should be defined in only one place, thus add new
serial-peripheral-props.yaml schema with definition of common properties
for UART-connected devices (children of UART controller): current-speed
and max-speed.  The schema can be referenced by individual devices using
these properties.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240811-dt-bindings-serial-peripheral-props-v1-2-1dba258b7492@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>