arm64: dts: imx8mn: put USB controller into power-domains

Now that we have support for the power domain controller on the i.MX8MN,
we can put the USB controller in the respective power domain to allow
it to power down the PHY when possible.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>