ENGR00219193 improve gpu3d core clock to 528M

ENGR00219193 improve gpu3d core clock to 528M

The original 528M setting is invalid and become 396M actually,
Change gpu3d core clock parent to 594_PFD to enable 528M setting.

Benchmark performance are improved with clock change on i.MX6DL:
     Basemark2: 5.85 --> 7.66
     Nenamark2: 23.7 --> 27.4
     Quadrant 3d: 2186 --> 2270

Signed-off-by: Xianzhong <b07117@freescale.com>