Add USB support to Barebox bootloader in SAMA5D3 Xplained board -- 2

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.