char/ipmi: add documentation for sysfs interface

This is an attempt to document the sysfs interface for the IPMI drivers.
Descriptions were collected from v2.0 of the IPMI specification and from
code comments.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>