powerpc: MPC8548CDS: Compile legacy PCIe routines conditionally

Compile the legacy PCIe initialization routines only when
DM_PCI is not enabled.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>