ARM720: remove exports and forward decls

ARM720: remove exports and forward decls

Unneeded exports cause confusion about the module interfaces.
Make all 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.

Remove unneeded indirection for the write_memory() method.  Make
a table static, remove a can't-happen case with nasty exit().

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