ARM: dts: stm32: enable RTC on stm32f429-disco

This patch enables RTC on stm32f429-disco with LSI as clock source because
X2 crystal for LSE is not fitted by default.

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