ARM: dts: imx6: RDU2+: add assigned clocks for GPU 3D

This downclocks the shader domain from 720MHz to 594MHz, which gets rid
of brown-outs due to excessive current draw on the PMIC switchers. The
board has an unchanged power tree from the Quad design, which isn't able
to cope with higher current needs of the QuadPlus GPU at full speed.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>