dt-bindings: serial: 8250: update TX FIFO trigger level

Remove data type from tx-threshold trigger level as defined now as a
serial generic property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210413174015.23011-4-erwan.leray@foss.st.com


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