ASoC: audio-graph: add graph_parse_mclk_fs()

It is parsing mclk_fs at many places, but it should be
same operation. This patch adds graph_parse_mclk_fs()
and parse it.

This patch also renames similar function graph_get_conversion()
to graph_parse_convert().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>