[DOCS] fast_model_boot.S: Optionally pass a fixed-up device-tree blob

As Xvisor arm32ve port now supports boot-loader provided device-tree
blob, we add an option to pass the same in fast_model_boot.S (as
-DTB=<path-to-dtb>). Please note that no dtb fixup is done.

Signed-off-by: Sukanto Ghosh <sukantoghosh@gmail.com>