arm64: dts: imx8mm: imx8mn: imx8mp: imx8mq: Replace opp-xM with opp-x000000

Fix the following dtbs_check warning on all of i.MX8M variants:
"
opp-table: Unevaluated properties are not allowed ('opp-25M', 'opp-100M', 'opp-750M' were unexpected)
"
Using the following command:
"
$ sed -i '/opp-[0-9]\+M/ s@M {@000000 {@' arch/arm64/boot/dts/freescale/imx8m*
"

The Documentation/devicetree/bindings/opp/opp-v2-base.yaml expects the OPP
subnode names to be full frequency listings in Hz without unit suffixes.
Only the i.MX8M DTs are affected per "git grep 'opp-[0-9]\+M'", so fix them.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>