arm64: dts: imx8mp-debix: add USB host support

This adds support for the 4 USB3 host ports on the board, which are
connected to the i.MX8MP SoC via a Realtek RTS5411 hub.

As the schematic for the board is not available, I could not validate
that this really reflects the reality, but I modeled things after the
hacked in usage of the GPIOs in the downstream kernel.

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