ipmi: create hardware-independent softdep for ipmi_devintf

When a computer has an IPMI system interface, the device interface
is most probably also desired. Autoloading of ipmi_devintf currently
works only if ipmi_si has allocated a platform device. That doesn't
happen if the SI interface was detected e.g. via ACPI. But ACPI
detection is preferred these days, see e.g. kernel.org bug 46741.

This patch introduces a softdep in place of the existing modalias
for ipmi_devintf.

Signed-off-by: Martin Wilck <mwilck@suse.com>
Suggested-by: Takashi Iwai <tiwai@suse.com>

I moved this to ipmi_msghandler.c, so it works for all IPMI
interfaces.  Retested by Martin.

Tested-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>