ASoC: rt5616: add mclk property for rt5616 document

This patch add the mclk property for the CODEC driver,
since sometime the CODEC driver needs the clock enabled.

The system clock of ALC5616 can be selected from MCLK,
That's also make as the codec master clock provider,

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>