doc: Add minimal glossary ...

One of the current issues with the documentation is that terms and
abbreviations are frequently redefined. For example, we might have
a sentence like "... the SCP (System Control Processor) will ...".

These definitions might be repeated several times across pages, or
even within the same document. Equally, some of these abbreviations
are missed and are never expanded.

Sphinx provides a :term: keyword that takes some text and,
if that text is defined in a glossary document, links to its glossary
entry. Using this functionality will prevent repeated definitions
and will make the docs more maintainable by using a single
definition source.

The glossary added in this patch was created from a quick scrub of
the source code - there may be missing entries. The SDEI abbreviation
was used as an example.

Note that a global_substitutions file was created. This file contains
the RST 'replace' statements that convert plain text terms into linked
terms (by adding the ':term:' keyword to them). An example is:

.. |TF-A| replace:: :term:`TF-A`

The 'rst_prolog' variable in conf.py is used to inject this list of
replacements into each page. Terms must be surrounded with the pipe
character to be turned into links - this means that we can still
prevent certain terms from being linked if we don't want them to be.

Change-Id: I87010ed9cfa4a60011a9b4a431b98cb4bb7baa28
Signed-off-by: Paul Beesley <paul.beesley@arm.com>