ACPI: IPMI: Add helper to wait for when SMI is selected

ACPI: IPMI: Add helper to wait for when SMI is selected

On Dell servers, many APCI methods of acpi_power_meter module evaluate
variables inside IPMI region, so the region handler needs to be
installed. In addition to that, the handler needs to be fully
functional, and that depends on SMI being selected.

So add a helper to let acpi_power_meter know when the handler is
installed and ready to be used.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20240320084317.366853-1-kai.heng.feng@canonical.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>