scsi: docs: include SCSI Transport SRP diagram at the doc body

Instead of having a separate makefile, and be alone, group
it at the SCSI documentation and make it being built as part
of docs makefile.

Link: https://lore.kernel.org/r/419c455fb40c9a1e85cc9a654a7fdb07aeeccf71.1583136624.git.mchehab+huawei@kernel.org


Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>