FWU: Add FWU metadata structure and driver for accessing metadata

In the FWU Multi Bank Update feature, the information about the
updatable images is stored as part of the metadata, which is stored on
a dedicated partition. Add the metadata structure, and a driver model
uclass which provides functions to access the metadata. These are
generic API's, and implementations can be added based on parameters
like how the metadata partition is accessed and what type of storage
device houses the metadata.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>