ASoC: kirkwood: change the compatible string of the kirkwood-i2s driver

The compatible string of the kirkwood-i2s driver was chosen as
"marvell,mvebu-audio". Using such a compatible string is not a good
idea, since "mvebu" is the name of a large family of SOCs, in which
new, unknown SOCs will be coming in the future. It is therefore
impossible to know what will be evolutions of this hardware block in
the next generations of the SOCs. For this reason, the recommandation
for compatible strings of on-SOCs devices has always been to use the
name of the oldest SOC that has the hardware block. New SOCs that have
an exactly compatible hardware block can reference it using the same
compatible string. See [1], [2] and [3] for various cases were this
suggestion was made, including from Rob Herring, a Device Tree binding
maintainer.

As an example, there are already small differences between current
generations:

 * On Kirkwood, only one interrupt is used for audio.
 * On Dove, two interrupts are used, one for audio data and one for
   error reporting.

In the near future, I'll be adding audio support to Armada 370, which
allows has the same hardware block (but maybe with minor variants).

Therefore, this patch changes the driver to accept
"marvell,kirkwood-audio" and "marvell,dove-audio" as compatible
strings instead of the too-generic "marvell,mvebu-audio". The reason
for the two different compatible strings is the difference in the
number of interrupts used by the two SOCs for audio.

This Device Tree binding has never been part of a Linux kernel stable
release so far, so it can be changed now without breaking backward
compatibility.

[1] http://lists.infradead.org/pipermail/linux-mtd/2012-March/040417.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2013-April/161065.html
[3] http://lists.infradead.org/pipermail/linux-arm-kernel/2012-March/087702.html

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