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>