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/configs/P2020RDB-PC_36BIT_NAND_defconfig b/configs/P2020RDB-PC_36BIT_NAND_defconfig
index 10c27e9..542fe36 100644
--- a/configs/P2020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P2020RDB-PC_36BIT_NAND_defconfig
@@ -46,6 +46,8 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nor0=fef000000.nor"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=fef000000.nor:256k(vsc7385-firmware),256k(dtb),4608k(kernel),9984k(fs),1280k(u-boot)"
+CONFIG_OF_CONTROL=y
+CONFIG_DEFAULT_DEVICE_TREE="p2020rdb-pc_36b"
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_FSL_ESDHC=y
 CONFIG_MTD_NOR_FLASH=y
@@ -68,4 +70,3 @@
 CONFIG_FSL_ESPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
-CONFIG_OF_LIBFDT=y