ACPI / IPMI: allow ACPI_IPMI with IPMI_SSIF

ACPI_IPMI driver currently depends on IPMI System Interface (IPMI_SI)
driver to be enabled. IPMI_SI driver only handles KCS, SMIC and BT BMC
interfaces.

IPMI_SSIF is an alternative BMC communication method. It allows BMC to
be accessed over an I2C bus instead of a standard interface.

Change the dependency to IPMI_HANDLER so that ACPI_IPMI works with all
IPMI providers.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>