NTB: Add device introspection sysctl hierarchy

This should export all of the same information as the Linux ntb_hw_intel
debugfs info file, but with a bit more structure, in the sysctl tree
rooted at 'dev.ntb_hw.<N>.debug_info'.

Raw registers are marked as OPAQUE because reading them on some hardware
revisions may cause a hard lockup (NTB errata). They can be read with
'sysctl -x dev.ntb_hw.<N>.debug_info.registers'. On Xeon platforms,
some additional registers are available under 'registers.xeon_stats' and
'registers.xeon_hw_err'. They are exported as big-endian values so that
the 'sysctl -x' output is legible.

Shrink the feature mask to 32 bits so we can use the %b formatter in
'debug_info.features'.

Sponsored by: EMC / Isilon Storage Division