mips: octeon: Fix Octeon DDR driver to use the correct struct

Don't use "platdata_auto_alloc_size" but "priv_auto_alloc_size" instead
to auto allocate the private data struct, which is referenced via
dev_get_priv() in this driver. This fixes an ugly bug detected while
trying to boot via SPI NOR.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>