powerpc: Enable device tree support for T2080RDB

Add device tree for T2080RDB board 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/T2080RDB_SDCARD_defconfig b/configs/T2080RDB_SDCARD_defconfig
index d40ae38..b7f6baa 100644
--- a/configs/T2080RDB_SDCARD_defconfig
+++ b/configs/T2080RDB_SDCARD_defconfig
@@ -40,6 +40,8 @@
 CONFIG_MTDIDS_DEFAULT="nor0=fe8000000.nor,nand0=fff800000.flash,spi0=spife110000.1"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=fe8000000.nor:1m(uboot),5m(kernel),128k(dtb),96m(fs),-(user);fff800000.flash:1m(uboot),5m(kernel),128k(dtb),96m(fs),-(user);spife110000.1:1m(uboot),5m(kernel),128k(dtb),-(user)"
 # CONFIG_CMD_IRQ is not set
+CONFIG_OF_CONTROL=y
+CONFIG_DEFAULT_DEVICE_TREE="t2080rdb"
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_FSL_CAAM=y
 CONFIG_FSL_ESDHC=y
@@ -63,4 +65,3 @@
 CONFIG_FSL_ESPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
-CONFIG_OF_LIBFDT=y