warp7: Fix dfu_alt_info setting after DM conversion

After DM conversion, U-Boot is larger than 512 KiB, so we need to increase
the "size" of the dfu_alt_info setting.

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