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>

Add documentation for CPU specific abstraction and Errata workarounds ...

This patch adds documentation for CPU specific abstraction in the firmware-
design.md and adds a new document cpu-errata-workarounds.md to describe
the cpu errata workaround build flags.

Change-Id: Ia08c2fec0b868a0a107d0264e87a60182797a1bd