mb/purism/librem_mini: move mainboard asl under variant

Upcoming librem_cnl variant(s) won't share mainboard asl.

Change-Id: I9c2c43e5ae5efe161cc2135c920a1b9dee5a1317
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50499
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/purism/librem_cnl/dsdt.asl

@@ -23,5 +23,5 @@ DefinitionBlock(
 
 	#include <southbridge/intel/common/acpi/sleepstates.asl>
 
-	#include "acpi/mainboard.asl"
+	#include <variant/acpi/variant.asl>
 }