habanalabs: prepare preboot stage to dynamic f/w load

Start the skeleton for the dynamic F/W load by marking current preboot
code path as legacy.

Signed-off-by: Ohad Sharabi <osharabi@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>