ASoC: Merge AT91 and AVR32 support into a single atmel architecture

The Ateml AT91 and AVR32 SoC share common IP for audio and can share the
same driver code using the atmel-ssc API provided for both architectures.
Do this, creating a new unified atmel ASoC architecture to replace the
previous at32 and at91 ones.

[This was contributed as a patch series for reviewability but has been
squashed down to a single commit to help preserve both the history and
bisectability.  A small bugfix from Jukka is included.]

Tested-by: Jukka Hynninen <ext-jukka.hynninen@vaisala.com>
Signed-off-by: Sedji Gaouaou <sedji.gaouaou@atmel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>