ARM: dts: imx51-babbage: Pass the USB PHY regulator

The USB PHYs are supplied via MC13892 VUSB regulator rail,
so properly describe this in the device tree.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>