ARM: dts: hummingboard: convert onboard audio to simple-audio-card

The HB onboard audio currently makes use of the imx-audio-sgtl5000
binding. This binding does not support auxiliary audio devices such
as external amplifiers. The simple-audio-card binding does support
this property which allows systems incorporating the HB to add an
auxiliary device that's attached to the HB audio jack with an
overlay. Convert the HB onboard audio to use simple-audio-card for
this additional flexibility.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>