ARM: dts: stm32mp13: activate led on STM32MP13F-DK

Activate the led managed in stm32mp1 board for U-Boot indication
in STM32MP13F-DK device tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>