bindings: sound: davinci-mcasp: Add support for optional auxclk-fs-ratio

When McASP is bus master it's reference clock (AUXCLK) might not be a
static clock, but running at a specific FS ratio.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>