southbridge/hudson: Add support for ACPI enable/disable via SMI

This enables the ACPI SMI command port in the FADT table, and sets up
the hardware accordingly. If we have SMI enabled, then we don't set
the SCI_EN bit at boot, causing the OS to send the ACPI_ENABLE
command, as required by the ACPI spec. This gives us a chance to hook
into the mainboard_smi_apmc() handler.

Change-Id: Ib4c63d55b3132578dcae48bfe2092d4ea35821dd