Create Table of Content links in markdown files ...

Fixes arm-software/tf-issues#276

Miscellaneous documentation fixes ...

This patch gathers miscellaneous minor fixes to the documentation, and comments
in the source code.

Change-Id: I631e3dda5abafa2d90f464edaee069a1e58b751b
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>
Co-Authored-By: Dan Handley <dan.handley@arm.com>

Document design of the Interrupt Mangement Framework ...

This patch adds documentation that describes the design of the Interrupt
management framework in the ARM Trusted Firmware. The porting-guide.md has also
been updated to describe the interface that should be implemented by each
platform to support this framework.

Change-Id: I3eda48e5c9456e6a9516956bee16a29e366633b7
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>