dts: nios2: intel: Move SoC devicetree includes under a vendor directory

Cleans up SoC devicetree include file locations to follow the convention
of dts/<arch>/<vendor>/

Signed-off-by: Maureen Helm <maureen.helm@intel.com>

boards/nios2/altera_max10/altera_max10.dts

@@ -2,7 +2,7 @@
 
 /dts-v1/;
 
-#include <nios2f.dtsi>
+#include <intel/nios2f.dtsi>
 
 / {
 	model = "altera_max10";

boards/nios2/qemu_nios2/qemu_nios2.dts

@@ -2,7 +2,7 @@
 
 /dts-v1/;
 
-#include <nios2-qemu.dtsi>
+#include <intel/nios2-qemu.dtsi>
 
 / {
 	model = "qemu_nios2";