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

Former NTB Performance driver could only work with NTB devices, which
got Scratchpads available and had just two ports. Since there are
devices, which don't have Scratchpads and got more than two peer
ports, the performance measuring tool needs to be rewritten. This
patch adds the ability to test any available NTB peer.
Additionally it allows to set NTB memory windows up using any
available data exchange interface: Scratchpad or Message registers.
Some cleanups are also added here.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>