FWU: doc: Add documentation for the FWU feature

Add documentation for the FWU Multi Bank Update feature. The document
describes the steps needed for setting up the platform for the
feature, as well as steps for enabling the feature on the platform.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>