Platform: drivers and external components

This includes all the platform drivers required by Trusted Firmware M.
The 'ext' directory contains code for target support which has been
brought in from other project.

Change-Id: Iaf4659e1d9b2ad4c662a2e5566571657042b5f2e
Signed-off-by: Abhishek Pandit <abhishek.pandit@arm.com>