ixgbe: Mailbox header and code module

The 82599 virtual function device and the master 82599 physical function
device implement a mailbox utility for communication between the devices
using some SRAM scratch memory and a doorbell/answering mechanism enabled
via interrupt and/or polling.  This C module and accompanying header
file implement the base functions for use of this feature.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>