Add basic support for codec-to-codec link in the axg sound card.
The cpu side of these links is expected to properly set the hw_params
and format of the link.

ATM, only the tohdmitx glue is supported but others (like the
internal DAC glue) should follow.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>