ASoC: codecs: ad193x: Add support to disable on-chip PLL

ASoC: codecs: ad193x: Add support to disable on-chip PLL

The on-chip PLL can be disabled if on the MCLKI pin we have an external
clock at 512 x fs. This clock can be used as direct internal clock for
ADCs or DACs.
To support this, we add an extra clock id that can be configured
using the set_sysclk() callback.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>