[SCSI] megaraid_sas: Add support to differentiate between iMR vs MR Firmware

Add support to differentiate between iMR(no external memory) and MR(with
external memory) controllers.

Signed-off-by: Sumit Saxena <sumit.saxena@lsi.com>
Signed-off-by: Kashyap Desai <kashyap.desai@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>