ARM: dts: am335x-sl50: use audio-graph-card for sound

audio-graph-card is recommended for audio bindings. Let's change to it and
improve the support by adding the Amplifier configuration and the clock
enable control.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>