ARM: dts: imx: add usb alias

[ Upstream commit 5c8b3b8a182cbc1ccdfcdeea9b25dd2c12a8148f ]

Add usb alias for bootloader searching the controller in correct order.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Stable-dep-of: ee70b908f77a ("ARM: dts: nxp/imx6sll: fix wrong property name in usbphy node")
Signed-off-by: Sasha Levin <sashal@kernel.org>