ASoC: hdmi-codec: add channel mapping control

Add user interface to provide channel mapping.
In a first step this control is read only.

As TLV type, the control provides all configuration available for
HDMI sink(ELD), and provides current channel mapping selected by codec
based on ELD and number of channels specified by user on open.
When control is called before the number of the channel is specified
(i.e. hw_params is set), it returns all channels set to UNKNOWN.
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>