arm/dt: Add skeleton dtsi file

Contains the bare minimum template required to boot with the device tree.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

arch/arm/boot/dts/skeleton.dtsi

+/*
+ * Skeleton device tree; the bare minimum needed to boot; just include and
+ * add a compatible value.  The bootloader will typically populate the memory
+ * node.
+ */
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+	chosen { };
+	aliases { };
+	memory { device_type = "memory"; reg = <0 0>; };
+};