ARM: ADIv5 symbol and comment cleanup

Instead of magic numbers, use their AP_REG_* constants.  Rename
the ROM address symbol as BASE to match ARM's documentation.

Comment various other symbols in the header; add some missing ones.
Remove an unused struct.  Add some doxygen for stuff including the
DAP structure and initialization.

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