ASoC: atmel-classd: add the Audio Class D Amplifier

Add driver for the digital imput to PWM output stereo
class D amplifier. It comes with filter, digitally
controlled gain, an equalizer and a dmphase filter.

Signed-off-by: Songjun Wu <songjun.wu@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>