ARM: dts: stm32: lxa-tac: disable the real time clock

The RTC was enabled under the false assumption that the SoM already
contains a suitable 32.768 kHz crystal.

It does however not contain such a crystal and since none is fitted
externally to the SoM the RTC can not be used on the hardware.

Reflect that in the devicetree.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>