arm64: dts: imx8mp: configure GPU and NPU clocks to overdrive rate

A lot of other clocks on the i.MX8MP, including the DRAM set up by the
bootloader are already running at overdrive clock rates. While this is a
deviation from the configuration of other i.MX8M* family SoCs, overdrive
is the default for most i.MX8MP boards and only some special purpose
boards will choose to run the SoC at nominal drive rates. Up the GPU and
NPU clock rates to their overdrive level to be consistent with other
clocks set up in the dtsi.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>