Audio HAL: Add support for Tungsten audio amplifier

The Steelhead device has three audio outputs:
0: OMAP4 HDMI
1: S/PDIF
2: TI TAS5713 Audio Amplifier.

This commit replaces the current audio HAL with a new one that supports
the audio amplifier.  In order to support all three audio output devices
on Steelhead, more work is required.