mpt2sas: Move Gen2 HBA's device registration to a separate file

1. Create a mpt2sas_module.c file for mpt2sas where GEN2 HBA devices
register with PCI, SML, IOCTL subsystems.

2. Updated the Makefile to use the object files from mpt3sas folder.

3. Defined a compilation flag SCSI_MPT2SAS which can be used to not
include those sections of code from mpt3sas driver which are not
required for mpt2sas driver.

4. Inherited automatic diag buffer feature from mpt3sas driver.

Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>