ASoC: topology: Add definitions for mclk_direction values

Current comment makes not clear the direction of mclk. Previously, similar
description caused a misunderstanding for bclk_master and fsync_master.

This commit solves the potential confusion the same way it is solved for
bclk_master and fsync_master.

Signed-off-by: Kirill Marinushkin <k.marinushkin@gmail.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Mark Brown <broonie@kernel.org>
Cc: Pan Xiuli <xiuli.pan@linux.intel.com>
Cc: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Cc: linux-kernel@vger.kernel.org
Cc: alsa-devel@alsa-project.org
Signed-off-by: Mark Brown <broonie@kernel.org>