ASoC: rt5640: enable MCLK detection

There is a power saving mechanism in rt5640. It will turn off some
unused power when MCLK is not present. We call that "MCLK detection"
and it should be enabled by default.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>