samples: drivers: led pwm running on 2 pwm leds on nucleo_u575

Add the overlay file to run the samples/drivers/led_pwm
on the nucelo_u575zi_q board.
Two green leds are used on pc7 and pb7 corresponding to pwm
channels of timers3 and timers4.
The leds node has to be disabled to let the pin for the pwm nodes.

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