Add USB support to Barebox bootloader in SAMA5D3 Xplained board

We are running the Barebox bootloader in an Atmel SAMA5D3 Xplained board. We need to add USB support to Barebox on this board, so that it is possible to mount a pen drive from within Barebox and read files from it. The end goal is to be able to do kernel/rootfs updates from a USB stick (Ethernet will not be available). We believe this should not be too difficult: - USB support in Barebox should be in good shape in general. Some Barebox users report that they use the EHCI driver regularly on other boards. - Barebox already has support for USB on Atmel SOCs (drivers ehci-atmel.c and at91-ohci.c) We think that probably this is just a matter of some initialisation that is currently missing from the Barebox SAMA5D3 Xplained board code. Also, the latest versions of U-boot (Barebox's predecessor) have working USB support for the SAMA5D3 Xplained. So it should be possible to check what U-Boot is doing, and see what is missing from Barebox.