ARM: dts: stm32: set HSE_RTC clock frequency to 1 MHz on stm32f746

This patch set HSE_RTC clock frequency to 1 MHz, as the clock supplied to
the RTC must be 1 MHz.

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