dt-bindings: gnss: reference serial-peripheral-props.yaml

The "current-speed" property is not a common property for all GNSS
devices, but only to these connected with serial.  Drop the property
from the common GNSS properties schema and instead reference common
serial properties schema (for children of UART controllers).

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-v1-4-1dba258b7492@linaro.org


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