tools/memory-model: docs/references: Remove broken link to imgtec.com

MIPS documents are not provided at imgtec.com any more.
Get rid of useless link.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>