acpi: Add IPMI table entries

acpi: Add IPMI table entries

Use the ACPI table construction tools to create an ACPI entry
for IPMI.  This adds a function called build_acpi_ipmi_devices
to add an DSDT entry for IPMI if IPMI is compiled in and an
IPMI device exists.  It also adds a dummy function if IPMI
is not compiled in.

This conforms to section "C3-2 Locating IPMI System Interfaces in
ACPI Name Space" in the IPMI 2.0 specification.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>