boards: stm32f4_disco:  added missing LED aliases
boards: stm32f4_disco:  added missing LED aliases

added three missing LEDS for stm32f4_disco, in order to run the
samples/basic/disco application on the stm32f4_disco

Signed-off-by: Jan Sturm <aa375376a3dab38a9f72e1bb67eaf1252e30f5f3@googlemail.com>

Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr