ASoC: kirkwood: enable Kirkwood driver for mvebu platforms

The audio unit found in the Armada 370 SoC is similar to the one used
in the Marvell Kirkwood and Marvell Dove SoCs. Therefore, this commit
allows the Kirkwood audio driver to be built on mvebu platforms, and
adds an additional compatible string to identify the Armada 370
variant of the audio unit.

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