mpt3sas: Add dummy Kconfig option for backwards compatibility

The mpt2sas driver was recently folded into mpt3sas to reduce code
duplication.

To avoid problems for people that only have CONFIG_SCSI_MPT2SAS in their
.config we introduce a dummy option that will select MPT3SAS if MPT2SAS
was previously enabled.

This is a temporary measure and we will deprecate this config option in
4.6.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: James Bottomley <James.Bottomley@hansenpartnership.com>
CC: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>