arm64: dts: imx8mm: put USB controllers into power-domains

Now that we have support for the power domain controller on the i.MX8MM
we can put the USB controllers in their respective power domains to allow
them to power down the PHY when possible.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>