powerpc: Enable device tree support for P1020RDB

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/P1020RDB-PC_SPIFLASH_defconfig b/configs/P1020RDB-PC_SPIFLASH_defconfig
index 8b46b08..fe14968 100644
--- a/configs/P1020RDB-PC_SPIFLASH_defconfig
+++ b/configs/P1020RDB-PC_SPIFLASH_defconfig
@@ -38,6 +38,8 @@
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_OF_CONTROL=y
+CONFIG_DEFAULT_DEVICE_TREE="p1020rdb-pc"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_FSL_ESDHC=y
 CONFIG_MTD_NOR_FLASH=y
@@ -58,4 +60,3 @@
 CONFIG_FSL_ESPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
-CONFIG_OF_LIBFDT=y