xtensa: xtfpga: add lx200 SMP DTS and defconfig

This config allows running SMP-enabled bitstream on LX200 board.
NFS or FLASH rootfs, minimal debug, up to 4 cores.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/boot/dts/lx200mx.dts

@@ -0,0 +1,16 @@
+/dts-v1/;
+/include/ "xtfpga.dtsi"
+/include/ "xtfpga-flash-16m.dtsi"
+
+/ {
+	compatible = "cdns,xtensa-lx200";
+	memory@0 {
+		device_type = "memory";
+		reg = <0x00000000 0x06000000>;
+	};
+	pic: pic {
+		compatible = "cdns,xtensa-mx";
+		#interrupt-cells = <2>;
+		interrupt-controller;
+	};
+};