ARM: dts: imx28-evk: add usb devices

- add usb phy devices
- add usb controller devices
- add usb vbus regulators

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Tested-by: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>