rocker: add register programming guide

This is the register programming guide for the Rocker device.  It's intended
for driver writers and device writers.  It covers the device's PCI space,
the register set, DMA interface, and interrupts.
Signed-off-by: NScott Feldman <sfeldma@gmail.com>
Signed-off-by: NJiri Pirko <jiri@resnulli.us>
Message-id: 1426306173-24884-4-git-send-email-sfeldma@gmail.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>