ARM: dts: am437x-sk-evm: Switch using simple-audio-card for audio support

The sound support consist only Headset output on the board and can be
handled by "simple-audio-card".
Also enable the Line-In route as the board has support for it.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>