Currituck doesn't need nor use SDR so aborting the pci setup if there is
no sdr-base would be bad.

Add a flag to ppc4xx_pciex_hwops for the backends to state if they need
SDR and then only complain and abort if they do and it's not found in
the device tree.
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Josh Boyer <jwboyer@gmail.com>