fm10k: Add support for mailbox

This patch adds generic mailbox support.  The general idea of the mailboxes
is to use a pair of ring buffers, one for request, one for response to send
data between the local driver and some remote entity be it the PF of the
Switch Manager.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>