arm64: dts: librem5-devkit: add the sgtl5000 i2c audio codec

Describe the sgtl5000 of the librem 5 devkit in devicetree.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>