[MTD] [NAND] remove __PPC__ hardcoded address from DiskOnChip drivers

Such a hardcoded address can cause a checkstop or machine check if
the driver is in the kernel but the address is not acknowledged.

Both drivers allow an address to be specified as either a module
parameter or config option.   Any future powerpc board should either
use one of these methods or find the address in the device tree.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>