warp7: Switch to DM USB

This commit switches to DM USB for warp7 and warp7_bl33 defconfigs.

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Signed-off-by: Joris Offouga <offougajoris@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>