ARM: dts: stm32: lxa-mc1: enable DDR50 mode on eMMC

The "eMMC high-speed DDR mode (3.3V I/O)" at 50MHz is supported on
the eMMC-interface of the lxa-mc1. Set it in the device tree to
benefit from the speed improvement.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Holger Assmann <h.assmann@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>