powerpc: Enable device tree support for P2020RDB

Add device tree for P1020RDB boards and enable CONFIG_OF_CONTROL
so that device tree can be compiled.
Update board README for device tree usage.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
diff --git a/arch/powerpc/dts/p2020-post.dtsi b/arch/powerpc/dts/p2020-post.dtsi
new file mode 100644
index 0000000..f20d1fa
--- /dev/null
+++ b/arch/powerpc/dts/p2020-post.dtsi
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0+ OR X11
+/*
+ * P2020 Silicon/SoC Device Tree Source (post include)
+ *
+ * Copyright 2013 Freescale Semiconductor Inc.
+ * Copyright 2019 NXP
+ */
+
+&soc {
+	#address-cells = <1>;
+	#size-cells = <1>;
+	device_type = "soc";
+	compatible = "fsl,p2020-immr", "simple-bus";
+	bus-frequency = <0x0>;
+
+	mpic: pic@40000 {
+		interrupt-controller;
+		#address-cells = <0>;
+		#interrupt-cells = <4>;
+		reg = <0x40000 0x40000>;
+		compatible = "fsl,mpic";
+		device_type = "open-pic";
+		big-endian;
+		single-cpu-affinity;
+		last-interrupt-source = <255>;
+	};
+};