fm10k: Add support for PF <-> VF mailbox

This patch adds support for the PF <-> VF mailbox.  It functions similar to
the PF <-> SM mailbox however there are several modifications made to
improve the reliability of the mailbox itself.  In addition the PF/VF
mailbox is much smaller an only supports a total size of 16 DWORDs vs the
1024 DWORDS provided for the PF/SM mailbox.

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