bus/fslmc: support scanned device count

FSLMC bus detects a multiple type of logical objects representing
components of the datapath.

Using the type of device, a newly introduced API
rte_fslmc_get_device_count can return the count of devices
scanned of that device type.

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>