boards: arm: stm32l475 board add nodes for timers15 and pwm leds

Add a new node to the DTS of the disco_l475_iot1 board for timers15
Output pin for the pwm channel1 is PB14 (D10 of arduino CN1)
Add the nodes for pwm leds corresponding to the pwm channels
of the timer 2 and timer 15.

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