ARM: dts: stm32: update rtc st,syscfg property on stm32f746

To fit with latest rtc driver updates, rtc st,syscfg property must contain
the control register offset of pwrcfg and the mask corresponding to the
DBP (Disable Backup Protection) bit.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>