ipmi: Convert the IPMI SI ACPI handling to a platform device

The IPMI SI driver was using direct PNP, but that was not really
ideal because the IPMI device is a platform device.  There was
some special handling in the acpi_pnp.c code for making this work,
but that was breaking ACPI handling for the IPMI SSIF driver.

So without this patch there were significant issues getting the
SSIF driver to work with ACPI.

So use a platform device for ACPI detection and remove the
entry from acpi_pnp.c.

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