imx-base.inc: Enable machine overrides extender for i.MX6

imx-base.inc: Enable machine overrides extender for i.MX6

This enables the machine overrides extender for the i.MX6 devices. It
maps (spaces are used to easy the read):

 - mx6q  ->        imxipu imxvpu imxgpu2d imxgpu3d
 - mx6dl -> imxpxp imxipu imxvpu imxgpu2d imxgpu3d imxepdc
 - mx6sx -> imxpxp               imxgpu2d imxgpu3d
 - mx6sl -> imxpxp               imxgpu2d          imxepdc
 - mx6ul -> imxpxp
 - mx7d  -> imxpxp                                 imxepdc

So it allows for reusing of overrides while keeps this control at the
BSP side.

Change-Id: Ie43aa497ce57b4a29f7b37a3eb941fb0d66691aa
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>