NTB: ntb_pp: Add full multi-port NTB API support

Current Ping Pong driver can't truly work with multi-port devices.
Additionally it requires the Scratchpad registers being available
on NTB device. This patches rewrites the driver so one would
perform the cyclic Ping-Pong algorithm around all the available
NTB peers and makes it working with NTB hardware, which doesn't
support Scratchpads, but such alternative as NTB Message register.
Additional cleanups are also added here.
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>