arm64: dts: imx8mn: remove "simple-bus" for anatop

Remove "simple-bus" compatible for device anatop,
since no child nodes exist under it and it is not
a populated bus.

Signed-off-by: Fancy Fang <chen.fang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>