sunxi: musb: Implement dfu_usb_get_reset()

This is necessary to distinguish between the "dfu-util --detach" and
the "dfu-util --reset" requests.

The default weak implementation of dfu_usb_get_reset() unconditionally
reboots the device, but we want to be able to continue the boot.scr
execution after writing the kernel, fdt and ramdisk to RAM via DFU.

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>