Revert "ACPI / AC: Remove AC's proc directory."

Revert commit ab0fd674 (ACPI / AC: Remove AC's proc directory.),
because some old tools (e.g. kpowersave from kde 3.5.10) are still
using /proc/acpi/ac_adapter.

Fixes: ab0fd674

 (ACPI / AC: Remove AC's proc directory.)
Reported-and-tested-by: Sorin Manolache <sorinm@gmail.com>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>