ASoC: ti: davinci-i2s: Use external clock to drive sample rate generator

McBSP's internal sample rate generator can be programed to be driven by
its internal clock or by an external clock source located on CLKS pin.
The external clock source case is not handled by the driver.

Handle an optional clock related to this external clock source. If
present, the driver uses the clock located on CLKS pin as input for the
sample rate generator. Thus, the external clock rate is used to compute
divisors. If this optional clock is not present, the sample rate
generator is driven by the McBSP's functional clock.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Link: https://msgid.link/r/20240402071213.11671-6-bastien.curutchet@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>