IPMI: Convert system interface defines to an enum

Convert the #defines for statistics into an enum in the IPMI system interface
and remove the unused timeout_restart statistic.  And comment what these
statistics mean.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>