eth: fbnic: Add FW communication mechanism

Add a mechanism for sending messages to and receiving messages
from the FW. The FW has fairly limited functionality, so the
mechanism doesn't have to support high message rate.

Use device mailbox registers to form two rings, one "to" and
one "from" the device. The rings are just a convention between
driver and FW, not a HW construct. We don't expect messages
larger than 4k so use page-sized buffers.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/172079937113.1778861.10669864213768701947.stgit@ahduyck-xeon-server.home.arpa


Signed-off-by: Jakub Kicinski <kuba@kernel.org>