ARM7/ARM9: remove old "debug commands"

ARM7/ARM9: remove old "debug commands"

Remove two commands that were documented as "debug commands"
and where "you probably don't want to use this".   We never
intended to support them, and at least one problem report
boiled down to using this when it shouldn't have been used.

Update the docs on the existing register commands to talk a
bit more about register access and cache behavior.  (Those
debug commands existed largely to *bypass* the cache.)  And
fix some minor doc goofs that snuck in with recent changes,
renaming "armv4_5" as "arm" and "arm9tdmi" as "arm9".

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