acpi, acpi_piix, vt82c686: factor out PM1a EVT logic

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: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>