boards: nucleo_l496zg: Add pwm mapping and RTC

Supported PWM at pins A0, D3, D5 and D6 which are also Arduino
compatible. Also the onboard red LED can be used with PWM.

The basic/{fade_led, blink_led} samples to apply to this board
seemlessly and operate on the onboard red LED.

Also supported RTC.

Signed-off-by: Ioannis Konstantelias <ikonstadel@gmail.com>