ARM11: minor cleanup, mostly ITR comments

ARM11: minor cleanup, mostly ITR comments

ITR register handling seemed to be giving me problems, so I updated
the comments to better say what the code is trying to do ... and to
note the preconditions (one of which seems to be an issue) as listed
in the ARM1136 TRM.

Also removed the unused "ARM11_TAP_DEFAULT" from the ITR scan code;
all the callers already specify an exit path, since this register
isn't usable with such vague semantics.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>