Merge changes from topic "gr/lts-doc" into integration

* changes:
docs: updates to LTS
docs: add inital lts doc

docs: add inital lts doc

Ref: https://linaro.atlassian.net/browse/TFC-669

The initial LTS document was created as pdf and was maintained in a
shared folder location, to avoid pdf getting lost and t

docs: add inital lts doc

Ref: https://linaro.atlassian.net/browse/TFC-669

The initial LTS document was created as pdf and was maintained in a
shared folder location, to avoid pdf getting lost and trying to find
where it is we decided to have LTS details part of docs in TF-A.

This patch directly reflects the data from pdf attached to TFC-669.
Any improvements or amends to this will be done at later phases based
on LTS maintainers comments and agreements.

Change-Id: I1434c29f0236161d2a127596e2cc528bf4cc3e85
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>

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