doc: Add Coding Guidelines document

doc: Add Coding Guidelines document

This content has been imported and adapted from the TF GitHub wiki
article 'ARM-Trusted-Firmware-Coding-Guidelines'.

The aim is to increase the visibility of the coding guidelines by
including them as part of the documentation that is within the TF
repository.

Additionally, the documentation can then be linked to by other
documents in the docs/ directory without worrying about broken links
to, for example, the external wiki.

Change-Id: I9d8cd6b5117b707c1a113baeba7fc5e1b4bf33bc
Signed-off-by: Paul Beesley <paul.beesley@arm.com>