ARM: dts: am57xx: cl-som-am57x: add analog audio support

Add analog audio DT nodes:

1. simple-audio-card node
2. wm8731 codec node
3. MCASP3 pinmux

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>