ASoC: spdif_transceiver: add output pin widget

CODECs without any outputs now remain powered down, which means any
paths to these codecs also remain powered down.

Add an always-enabled output pin widget to the spdif transceiver codec.
This enables DAPM to correctly identify that the spdif transceiver is
in use when playback is enabled, which will then allow DAPM to power up
any links from the CPU DAI to the S/PDIF transceiver.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>