boards: arm: stm32u575 nucleo add nodes for timers3 & 4 and pwm leds

Add and enable the timers3 and timers4 of the nucleo_u575zi_q.
Both PWM channel 2 of each timer have output pins connected to leds
on the board.
Add the nodes for pwm leds corresponding to the pwm channels
of the timer 3 and timer 4.

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