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/board/freescale/p1_p2_rdb_pc/README b/board/freescale/p1_p2_rdb_pc/README
index 26902de..b85cf02 100644
--- a/board/freescale/p1_p2_rdb_pc/README
+++ b/board/freescale/p1_p2_rdb_pc/README
@@ -48,7 +48,7 @@
 
 Device tree support and how to enable it for different configs
 --------------------------------------------------------------
-Device tree support is available for p1020rdb for below mentioned boot,
+Device tree support is available for p1020rdb and p2020rdb for below mentioned boot,
 1. NOR Boot
 2. NAND Boot
 3. SD Boot