The maintainers.rst file provides the list of all TF-A modules and their
code owners. As there are quite a lot of modules (and more to come) in
TF-A, it is sometimes hard to find the information.

Introduce categories (core code, drivers/libraries/framework, ...) and
classify each module in the right one.

Note that the core code category is pretty much empty right now but the
plan would be to expand it with further modules (e.g. PSCI, SDEI, TBBR,
...) in a future patch.

Change-Id: Id68a2dd79a8f6b68af5364bbf1c59b20c05f8fe7
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>