mailbox: Add new mailbox for gvm

Add new mailbox for gvm which will be used to siganl primary
host when gvm want to communicate with remote sub system. This
mailbox will use virtual IRQ line which will trigger interrupt
to primary host and primary host will then forward this interrupt
to corresponding remote sub system.

Change-Id: Ib9e10e26f0e636e9a3a4cbaf9676362f5574d681
Signed-off-by: Arun Prakash <app@codeaurora.org>