IPMI: Add the document description of ipmi_get_smi_info

Add the document description about how to use ipmi_get_smi_info.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Len Brown <len.brown@intel.com>