ALSA: add Intel HDMI LPE audio driver for BYT/CHT-T

On Baytrail and Cherrytrail, HDaudio may be fused out or disabled
by the BIOS. This driver enables an alternate path to the i915
display registers and DMA.

Although there is no hardware path between i915 display and LPE/SST
audio clusters, this HDMI capability is referred to in the documentation
as "HDMI LPE Audio" so we keep the name for consistency. There is no
hardware path or control dependencies with the LPE/SST DSP functionality.

The hdmi-lpe-audio driver will be probed when the i915 driver creates
a child platform device.

Since this driver is neither SoC nor PCI, a new x86 folder is added
Additional indirections in the code will be cleaned up in the next series
to aid smoother DP integration

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Jerome Anand <jerome.anand@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>