dt-bindings: arm: stm32: Add compatible strings for Protonic T1L boards

This boards are based on STM32MP151AAD3 and use 10BaseT1L for
communication.

- PRTT1C - 10BaseT1L switch
- PRTT1S - 10BaseT1L CO2 sensor board
- PRTT1A - 10BaseT1L multi functional controller

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>