arm64: dts: freescale: imx8mm-verdin: Fix GPU speed

The GPU clock was reduced on iMX8MM SOC device tree to prevent boards
that don't support GPU overdrive from being out of specification. However,
this caused a regression in GPU speed for the Verdin iMX8MM, which does
support GPU overdrive. This patch fixes this by enabling overdrive mode
in the SOM dtsi.

Fixes: 1f794d3e ("arm64: dts: imx8mm: Reduce GPU to nominal speed")
Signed-off-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>