acpi/core: always set SCI_EN when SMM isn't supported

acpi/core: always set SCI_EN when SMM isn't supported

If SMM is not supported, ACPI fixed hardware doesn't support
legacy-mode. ACPI-only platform. Where SCI_EN in PM1_CNT register is
always set.
The bit tells OS legacy mode(SCI_EN cleared) or ACPI mode(SCI_EN set).

With the next patch (setting fadt.smi_cmd = 0 when smm isn't enabled),
guest Linux tries to switch to ACPI mode, finds smi_cmd = 0, and then
fails to initialize acpi subsystem. This patch proactively fixes it.

This patch changes guest ABI. To keep compatibility, use
"smm-compat" introduced by earlier patch. If the property is true,
disable new behavior.

ACPI spec 4.8.10.1 PM1 Event Grouping
PM1 Eanble Registers
> For ACPI-only platforms (where SCI_EN is always set)

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-Id: <500f62081626997e46f96377393d3662211763a8.1613615732.git.isaku.yamahata@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>