mpi3mr: configure larger max I/O size if the HBA firmware supports it

The max I/O size that an mpi3mr HBA supports is reported in the IOCFacts
structure (with 0 representing the legacy max I/O size of 1 MB).
By default, set the max I/O size of devices attached to mpi3mr controllers
to the smaller of the HBA's max I/O size and the kernel's maxphys.
Allow this default to be overriden by a global tunable
"hw.mpi3mr.max_sgl_entries" or by a per-controller tunable
"dev.mpi3mr.N.max_sgl_entries".

Sponsored by: Netflix
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D49090

(cherry picked from commit f2605f67a13eb1f1427ee58a097791ca25359a5e)