MGS-2619 [#imx-188] Use imx-gpu-subsytem method to probe GPU Devices

MGS-2619 [#imx-188] Use imx-gpu-subsytem method to probe GPU Devices

Mainline kernel already implements each GPU device as separate node
and gpu-subsystem is being used.

This method will suit better for i.MX8QM where each GPU have different
power domains and need for a separate node. By this way calling of direct
SCFW API can be avoided and use the linux call.

Date: Feb 16, 2017
Signed-off-by: Prabhu Sundararaj <prabhu.sundararaj@nxp.com>