pwm: Add device tree binding document for IMG PWM DAC

Add binding document for IMG Pulse Width Modulator (PWM) DAC present on
the Pistachio SOC. The PWM DAC has four channels.

Signed-off-by: Naidu Tellapati <Naidu.Tellapati@imgtec.com>
Signed-off-by: Sai Masarapu <Sai.Masarapu@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>