NTB: switchtec_ntb: Update switchtec documentation with notes for NTB
The switchtec_ntb driver has a couple requirements on the switchec's
hardware configuration so we add these notes to the documentation.
Signed-off-by: Logan Gunthorpe <logang@deltatee.com> Reviewed-by: Stephen Bates <sbates@raithlin.com> Reviewed-by: Kurt Schwemmer <kurt.schwemmer@microsemi.com> Acked-by: Allen Hubbe <Allen.Hubbe@dell.com> Signed-off-by: Jon Mason <jdmason@kudzu.us>