dt-bindings: serial: add RX and TX FIFO properties

Add two optional DT properties to configure RX and TX FIFO thresholds:
- rx-threshold
- tx-threshold

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210413174015.23011-2-erwan.leray@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>