Rewrite Mailbox.cs to match TLUL spec - one mailbox with two endpoints.

The previous impl had two mailboxes, each with both "ends" connected to
the same register block. This was wrong - each "end" of each mailbox
fifo should be connected to different register blocks. This makes the
simulation slightly trickier, but saner from the user perspective.

Change-Id: I3699630ed86a4decb25d8611d1a8f2c57118ea98