ARM: dts: stm32: add blue led (Linux heartbeat) on stm32mp135f-dk

The blue led (Linux heartbeat) is connected to the PA14 pin of the
stm32mp135f-dk board.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>