meta-ktn-imx: Add machine tune includes for older and newer releases

Weaken the include for the path in Dunfell releases and add another
reference to the path in Kirkstone so we can be compatible with both
releases.

layers/meta-ktn-imx/conf/machine/include/ktn-mx6ul.inc

-require conf/machine/include/tune-cortexa7.inc
+include conf/machine/include/tune-cortexa7.inc
+include conf/machine/include/arm/armv7a/tune-cortexa7.inc
 
 MACHINEOVERRIDES =. "mx6:mx6ul:ktn-mx6ul:"
 

layers/meta-ktn-imx/conf/machine/include/ktn-mx8mm.inc

-require conf/machine/include/tune-cortexa53.inc
+include conf/machine/include/tune-cortexa53.inc
+include conf/machine/include/arm/armv8a/tune-cortexa53.inc
 
 MACHINEOVERRIDES =. "mx8:mx8m:mx8mm:ktn-mx8mm:"