New bus_mux component

[Ben Elliston]

   The following patch is for a new glue component: bus_mux which is a
   bus multiplexer. It supports one bus input and attaches the bus to one
   of two accessors.  The output accessor can be switched via a switch
   pin.  I have included the patch plus a new documentation file.  Ok to
   check in?

Also, you should consider including your new component in gluemonkey.exp and
running it under the monkey tester for a few days.

Ben