ACPI: Add top-level ASL

ACPI: Add top-level ASL

Objects that are created with acpigen need to be declared
with External () for the generation of dsdt.asl to pass
iasl without errors.

There are some objects that are common to all platforms,
and some that should be declared only conditionally.
Having a top-level ASL helps to achieve this.

Change-Id: Ibaf1ab9941b82f99e5fa857c0c7e4b6192c74330
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49794
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>