rtc: stm32: add Low Speed Clock Output (LSCO) support

RTC is able to output on a pin the "LSE" internal clock.

STM32 RTC is now registered as a clock provider.
It provides rtc_lsco clock, that means RTC_LSCO is output on either
RTC_OUT1 or RTC_OUT2_RMP, depending on pinmux DT property.
The clock is marked as CLK_IGNORE_UNUSED and CLK_IS_CRITICAL because
RTC_LSCO can be early required by devices needed it to init.

Add LSCO in pinmux functions.

Add "stm32_rtc_clean_outs" to disable LSCO. As RTC is part of "backup"
power domain, it is not reset during shutdown or reboot. So force LSCO
disable at probe.

Co-developed-by: Amelie Delaunay <[email protected]>
Signed-off-by: Amelie Delaunay <[email protected]>
Signed-off-by: Valentin Caron <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexandre Belloni <[email protected]>

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