Tidying up a few Sphinx warnings that had built-up over time.
None of these are critical but it cleans up the Sphinx output.

At the same time, fixing some spelling errors that were detected.

Change-Id: I38209e235481eed287f8008c6de9dedd6b12ab2e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

The documentation for Marvell platforms was not included in the
rendered document output until now because, while it was mostly
valid RST format, the files were saved with a .txt extension.

This patch corrects some RST formatting errors, creates a document
tree (index page) for the Marvell documents, and adds the Marvell
subtree to the main index.

Change-Id: Id7d4ac37eded636f8f62322a153e1e5f652ff51a
Signed-off-by: Paul Beesley <paul.beesley@arm.com>