mx53loco: Turn on VUSB regulator

On the mx53loco board with mc34708 PMIC it is necessary to turn on VUSB regulator
so that the mx53 USBH1 PHY receives the 3.3V voltage.

Tested by inserting a USB pen drive in the upper USB slot (USBH1) and then issued the
commands:

usb start

usb info

,which correctly detected and printed the USB pen drive information.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Jason Liu <r64343@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>