xilinx: zynqmp: Add usb dfu/thor distro boot support

In usb boot mode distro boot should select usb device as primary boot
device instead of usb host. So make usb dfu as primary boot device. But do
not list it in boot_targets as fallback option because it is not classic
mode for booting. Using 60s timeout by default should be enough time for
dfu-utils to start transaction. In case none needs this please change
timeout value in the command or disable CONFIG_DFU_TIMEOUT.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>