ARM: dts: imx6ul-geam: Remove invalid sgtl5000 property

As per sgtl5000.yaml, 'clock-names' is not a valid property.

Remove it to fix the following schema warning:

imx6ul-isiot-nand.dtb: codec@a: Unevaluated properties are not allowed ('clock-names' was unexpected)

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>