sunxi: Enable DFU for RAM

sunxi: Enable DFU for RAM

The DFU protocol implementation in U-Boot is much faster than the
FEL protocol implementation in the boot ROM on Allwinner devices.
Using DFU instead of FEL improves the USB transfer speed from
500-900 KB/s to 3.2-3.7 MB/s. This is particularly useful for
reducing the time needed for booting systems with large initrd
images.

FEL is still useful for loading the U-Boot bootloader and a boot
script, which may then activate DFU in the following way:

   setenv dfu_alt_info ${dfu_alt_info_ram}
   dfu 0 ram 0
   bootm ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr_r}

The rest of the files can be transferred to the device using the
"dfu-util" tool.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>