ASoC: hdmi-codec: Add ELD control

ALSA doesn't know about all the different compressed audio formats,
so there is no interface to let userspace enumerate the formats that
are supported by the connected sink. Exporting the raw ELD bytes to
userspace allows an application to select the appropriate audio format
depending on the current capabilities of the connected HDMI sink device.
Usually userspace then just pretends to ALSA that the data is in one of
the raw 16-bit PCM audio formats and relies on the IEC controls to tell
the sink how to interpret the data.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Tested-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>