bus/fslmc: support memory event callbacks for VFIO

bus/fslmc: support memory event callbacks for VFIO

VFIO needs to map and unmap segments for DMA whenever they
become available or unavailable, so register a callback for
memory events, and provide map/unmap functions.

Remove unneeded check for number of segments, as in non-legacy
mode this now becomes a valid scenario.

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>