powerpc: Enable device tree support for MPC8548CDS

Add device tree for MPC8548CDS board and enable CONFIG_OF_CONTROL
so that device tree can be compiled.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>