dt-bindings: rtc: stm32: describe pinmux nodes

STM32 RTC is capable to handle 3 specific pins of the soc (out1, out2,
out2_rmp) and to outputs 2 signals (LSCO, alarm-a).

This feature is configured thanks to pinmux nodes and pinctrl framework.
This feature is available with compatible st,stm32mp1-rtc and
st,stm32mp25-rtc only.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240722160022.454226-2-valentin.caron@foss.st.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>