arm64: dts: imx8mp: configure GPU and NPU clocks in nominal DTSI

Commit 255fbd9e ("arm64: dts: imx8mp: Add optional nominal drive
mode DTSI") added imx8mp-nominal.dtsi, which overrides all overdrive
clock rates in imx8mp.dtsi to the nominal rates.

At the same time, commit 9f7595b3 ("arm64: dts: imx8mp: configure
GPU and NPU clocks to overdrive rate") went in, which changed some
clock rates away from the nominal values.

Resolve the discrepancy by effectively reverting the changes in the
latter commit inside imx8mp-nominal.dtsi. This is required for proper
operation of the imx8mp-skov boards, which are currently
imx8mp-nominal.dtsi's only users and lets all other boards that don't
include it benefit from the new higher frequencies.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>