ipmi: Consolidate the adding of platform devices

ipmi: Consolidate the adding of platform devices

It was being done in two different places now that hard-coded devices
use platform devices, and it's about to be three with hotmod switching
to platform devices.  So put the code in one place.

This required some rework on some interfaces to make the type space
clean.

Signed-off-by: Corey Minyard <cminyard@mvista.com>