haven: Add haven doorbell driver

In haven one way to provide inter VMs(Virtual Machines)
communication is through the use of doorbell interrupts.
Individual VMs make use of these doorbells by calling
send and/or a receive primitives exposed by driver and
trigger an interrupt to each other and exchange the data.

Change-Id: I6d797d78cddc96a23680554e2b3fc1031446cbe5
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>
Signed-off-by: Elliot Berman <eberman@codeaurora.org>