rtc: stm32: get DBP register and mask from DT st, syscfg property

RTC driver should not be aware of the PWR registers offset and bits
position. Furthermore, we can imagine that DBP relative register and bit
mask could change depending on the SoC.
So this patch introduces 2 parameters, dbp_reg and dbp_mask, allowing to
get PWR_CR and PWR_CR_DBP from device tree. And it prepares next RTC
version, backup domain write protection is disabled only if needed.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>