ASoC: Intel: boards: sof-wm8804: support for Hifiberry Digiplus boards

Add a new machine driver to interface with WM8804. The code is based
on settings found in sound/soc/bcm/hifiberry-digi.c in the Raspberry
PI tree.

Tested on Up2 with Digi+ IO (capture+playback) and Digi+ PRO (playback
with two local oscillators supported). The codec is clock master in
both cases.

Capture support has known clocking restrictions: the transmitter needs
to be active for the clock recovery, the "Tx Source" mixer set to
"S/PDIF RX". Playback will only work while capture is active.

When Capture is not desired, or when there is no RX connector, the "Tx
Source" mixer should be set to "AIF"

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20200501151625.17820-1-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>