ASoC: cs42l51: fix mclk support

The MCLK clock is made optional for cs42l51 codec.
However, ASoC DAPM clock supply widget, expects the clock to be defined
unconditionally.
Register MCLK DAPM conditionally in codec driver,
depending on clock presence in DT.

Fixes: 5e8d63a7

 ("ASoC: cs42l51: add mclk support")
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>