boards: nucleo_wba55cg: Add timer3 and pwmleds configuration

Added timer3 and pwmleds configuration to the nucleo_wba55cg board DTS
file. This includes setting the prescaler, enabling the pwm3 node with the
appropriate pin control settings, and configuring the pwmleds node with
green_pwm_led.

Signed-off-by: Lubos Koudelka <lubos.koudelka@st.com>