For folk who don't know the ARM920 JTAG interface very well, the
two modes of scan chain 15 access to CP15 are confusing.

Make those parts of the ARM920 code less opaque, by:

 - Adding comments referencing the relevant parts of the TRM,
   catching up to similar updates in the User's Guide.

 - Replacing magic numbers in physical access clients with
   symbolic equivalents.

No functional change.

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