xpedite5370: Enable NAND command support

Use the MPC8572's eLBC to access 1 GB (or greater) onboard NAND flash
via the 'nand' command.

Previously, the XPedite5370's NAND chip selects were properly
configured, but NAND support was not enabled.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>