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>
14 files changed