arm: mach-k3: j721e: Rename BOOT_DEVICE_USB to BOOT_DEVICE_DFU

J721e does not support USB Host MSC boot, but only supports DFU boot.
Since BOOT_DEVICE_USB is often used for host boot mode and
BOOT_DEVICE_DFU is used for DFU boot, rename BOOT_DEVICE_USB macro to
BOOT_DEVICE_DFU

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>