rtc: stm32: add pinctrl and pinmux interfaces

STM32 RTC is capable to handle 3 specific pins of the soc.
"out1, out2 and out2_rmp". To handle this, we use pinctrl framework.
There is a single pin per group.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240722160022.454226-3-valentin.caron@foss.st.com


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