NTB: switchtec: Add NTB hardware register definitions

There are two additional regions: ctrl and dbmsg. The first is
for generic NTB control and memory windows. The second is for doorbells
and message registers. This patch also adds a number of related
constants for using these registers.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>