Add information about Juno in firmware-design.md

This patch reorganizes the firmware design guide to add information about the
port of the ARM Trusted Firmware to the Juno ARM development platform.

Change-Id: I0b80e2e7a35ccad1af2e971506cfb7fe505f8b84
Merge pull request #202 from achingupta/ag/fw-design-juno-update

Add information about Juno in firmware-design.md