[POWERPC] 4xx: Use machine_device_initcall() for warp_nand

With a multiplatform kernel, once built we always have warp_setup_nand_flash()
called and NDFC probed, no matter what machine we actually run on. This
potentially can cause problems (such as kernel crash), since NDFC is probed at
a warp-predefined address.

Using machine_device_initcall() NAND devices are registered if we run on a warp only.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>