sound: ASoC: add ASoC board driver for Armada 370 DB

This commit adds a simple ASoC board driver fo the Armada 370
Development Board, which connects the audio unit of the Armada 370 SoC
to the I2C-based CS42L51.

For now, only the analog audio input and output through the CS42L51
are supported, but a followup patch adds S/PDIF support to this
driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>