factor out ACPI PM1a EVT logic.
Later this will be used by ich9 acpi.

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Huacai Chen <zltjiangshi@gmail.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>