samples: drivers: led pwm running on 2 pwm leds on disco l475

Add the overlay file to run the samples/drivers/led_pwm
on the disco_l475_iot1 board. Two green leds are used on pa5 and pb14
corresponding on 2 pwm channels of timers2 and timers15.
The leds node has to be disabled to let the pin for the pwm nodes.

Signed-off-by: Francois Ramu <francois.ramu@st.com>