dt-bindings: serial: stm32: override FIFO threshold properties

dt-bindings: serial: stm32: override FIFO threshold properties

Override rx-threshold and tx-threshold properties:
- extend description
- provide default and expected values

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Changes in v2:
Change added properties naming and factorize it in serial.yaml as proposed
by Rob Herring.

Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210413174015.23011-3-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>