refactor(versal2): guard handoff logic w/ build flag

Prepare for environments where the Firmware Handoff (LibTL) submodule
may not be available. Wrap all Transfer List dependent logic in `#if
TRANSFER_LIST` guards, ensuring the platform can build and run without
the submodule.

This is useful for builds not integrating the firmware handoff
mechanism.

Change-Id: Ia34bc0f4d352a3014c71eda6589c0f3e0a107ca0
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
2 files changed