ARM: dts: imx6sx: Complete the PXP support

According to Documentation/devicetree/bindings/media/fsl-pxp.txt,
only one PXP clock needs to be described and it should be named
"axi".

Also pass the compatible string as suggested in the bindings doc.

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