ASoC: sun4i-i2s: Add mclk enable regmap field

The location of the mclk output enable bit is different on newer
SoCs. Use a regmap field to enable it.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>