doc: Reformat platform port documents ...

The platform port documents are not very standardised right now and
they don't integrate properly into the document tree so:

1) Make sure each port has a proper name and title (incl. owner)
2) Correct use of headings, subheadings, etc in each port
3) Resolve any naming conflicts between documents

Change-Id: I4c2da6f57172b7f2af3512e766ae9ce3b840b50f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

docs: warp7: Update WaRP7 description for TBB ...

This patch updates the WaRP7 build descriptions for booting WaRP7 in
Trusted Board Boot mode. TBB is the only mode we really intend to support
for this board so rather than maintain documentation for the old way of
doing it, this patch updates the description for TBB mode only.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

docs: warp7: Add description for the i.MX7 WaRP7 platform ...

This patch describes the boot-flow and building of the WaRP7 TF-A port.
What it describes is booting and unsigned TF-A.

A very brief section has been added on signing BL2 which is in no-way
comprehensive. For a comprehensive description of the signing process try
the Boundary Devices blog on the matter.

https://boundarydevices.com/high-assurance-boot-hab-dummies/

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>