Simplify Firmware Design document

Simplify Firmware Design document

The Firmware Design document is meant to provide a general overview
of the Trusted Firmware code. Although it is useful to provide some
guidance around the responsibilities of the platform layer, it should
not provide too much platform specific implementation details. Right
now, some sections are too tied to the implementation on ARM
platforms. This makes the Firmware Design document harder to digest.

This patch simplifies this aspect of the Firmware Design document.
The sections relating the platform initialisations performed by the
different BL stages have been simplified and the extra details about
the ARM platforms implementation have been moved to the Porting Guide
when appropriate.

This patch also provides various documentation fixes and additions
in the Firmware Design and Platform Porting Guide. In particular:

 - Update list of SMCs supported by BL1.

 - Remove MMU setup from architectural inits, as it is actually
   performed by platform code.

 - Similarly, move runtime services initialisation, BL2 image
   initialization and BL33 execution out of the platform
   initialisation paragraph.

 - List SError interrupt unmasking as part of BL1 architectural
   initialization.

 - Mention Trusted Watchdog enabling in BL1 on ARM platforms.

 - Fix order of steps in "BL2 image load and execution" section.

 - Refresh section about GICv3/GICv2 drivers initialisation on
   ARM platforms.

Change-Id: I32113c4ffdc26687042629cd8bbdbb34d91e3c14