arm64: dts: imx8mp-nominal: Explicitly configure nominal VPU clocks

In preparation for increasing the default VPU clocks to overdrive,
configure the nominal values first to avoid running the nominal
devices out of spec when imx8mp.dtsi is changed.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>