arm64: dts: imx8mq: fix GPU clock frequency

v2 of "clk: imx: Refactor entire sccg pll clk" dropped the implicit
reparenting of the PLL output from the bypass clock to the real
PLL. The commit introducing the GPU node had only been tested against
v1 of this patch. Without an explicit reparent to the real PLL the
GPU is stuck at the bypass clock rate of 25MHz, serverly hampering
performance.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>