ipmi: introduce an ipmi_bmc_sdr_find() API

This patch exposes a new IPMI routine to query a sdr entry from the
sdr table maintained by the IPMI BMC simulator. The API is very
similar to the internal sdr_find_entry() routine and should be used
the same way to query one or all sdrs.

A typical use would be to loop on the sdrs to build nodes of a device
tree.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
