armv8: ls1028a: use the official compatible string for the GPU

armv8: ls1028a: use the official compatible string for the GPU

There is no "fsl,ls1028a-gpu" compatible string. It is solely for the
proprietary driver which will never be open source. Lately, linux gained
support for the open source etnaviv driver for the GPU (although there
is still support for the DisplayPort PHY missing to get actual graphics
output). Thus, instead of supporting some proprietary driver, switch
over to the open source one, which also have an official device tree
binding.

Cc: Andy Tang <andy.tang@nxp.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>