ARM: imx: fix typo in gpu3d_shader_sels

There is no clock pll2_pfd9_720m.  Instead it should be pll3_pfd0_720m.
Fix the typo in gpu3d_shader_sels.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>