ARM: zynq: Added USB host support for zynq boards

Added configs to support USB host for zynq boards.
Also added a command usbboot to boot from usb.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>