ASoC: TI AESS: add autogating-enable function, callable from architecture code

Add a basic header file for the TI AESS IP block, located in the OMAP4
Audio Back-End subsystem.

Currently, this header file only contains a function to enable the
AESS internal clock auto-gating.  This will be used by a subsequent
patch to ensure that the AESS won't block the entire chip
low-power-idle mode.  We wish to be able to place the AESS into idle
even when no AESS driver has been compiled in.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Liam Girdwood <lrg@ti.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Péter Ujfalusi <peter.ujfalusi@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>