mpsutil/mprutil: add flash subcommand

the flash subcommand allows to save/update firmware and bios for LSI Fusion-MPT
2/3 controllers (mps(4) and mpr(4))

Tested by: allanjude
Reviewed by: wblock (manpage)
Relnotes: yes
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D4026