ARM: dts: stm32: Add led-blue for stm32mp157c-ed1-u-boot

The blue led is used to indicate U-Boot entering / exit indication
then Linux heartbeat.

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