isp(4): Add support to read contents of the FLT (flash layout table)

The FLT is like a TOC for the flash area and contains entries for every flash
region with start/end address, size and flags.
Start using NVRAM addresses from FLT instead of hardcoded ones for ISP28xx
based HBAs.

The FLT should be available on earlier HBAs too, probably since ISP24xx based.
This needs further investigation and testing.

PR: 271062
Reviewed by: imp, mav
Sponsored by: Technical University of Munich
Pull Request: https://github.com/freebsd/freebsd-src/pull/726

(cherry picked from commit 27b4a1b7e5b871c248dc501aa359c93f7263b074)