doc: Add instructions to clean the flash for Secure FWU

In Secure Firmware Update, if the user runs the demo and tries
to run the validation tests or vice versa, they fail because
the Firmware has already been updated.

Add instructions to first build a clean flash before running the
demo or the validation tests.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>