ARM920: remove exports and forward decls

Unneeded exports cause confusion about the module interfaces.
Make most functions static.  Add a short header comment.

The forward decls are just code clutter; remove them, by moving
their references after definitions.  This is another file which
never needed even one internal forward declaration.

This saved almost 900 bytes of code on x86_32; it seems the
compiler can leverage its knowledge that these functions are
not called from the outside world...

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