ASoC: dt-bindings: davinci-mcbsp: Add optional clock

The McBSP uses an internal sample rate generator to provide bit clock
or frame clock. This sample rate generator can be programmed to be
driven by McBSP's internal clock source or by an external clock source
(located on CLKS pin). The external clock source is not described in
the bindings.

Add an optional clock item that allows to select an external clock as
sample rate generator's input.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Link: https://msgid.link/r/20240402071213.11671-3-bastien.curutchet@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>