eth: fbnic: Implement Tx queue alloc/start/stop/free

Implement basic management operations for Tx queues.
Allocate memory for submission and completion rings.
Learn how to start the queues, stop them, and wait for HW
to be idle.

We call HW rings "descriptor rings" (stored in ring->desc),
and SW context rings "buffer rings" (stored in ring->*_buf union).

This is the first patch which actually touches CSRs so add CSR
helpers.

No actual datapath / packet handling here, yet.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/172079938724.1778861.8329677776612865169.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Jakub Kicinski <kuba@kernel.org>