Provide basic documentation on the ARM ETM and ETB trace commands.

Fix minor goofs in registration of the ETM commands; and whitespace
issues in the proof-of-concept oocd_trace code.  (Plus include a
ref to Dominic's email saying that it's just proof-of-concept code.)

Note that I'm still not sure whether the ETM support works.  But
documenting how it's expected to work should help sort out which
behaviors are bugs, which will help get bugs patched.

ZW: whitespace changes were split out of this patch but will follow.


git-svn-id: svn://svn.berlios.de/openocd/trunk@1945 b42882b7-edfa-0310-969c-e2dbd0fdcd60