powerpc/axonram: Enable partitioning of the Axons DDR2 DIMMs

DDR2 memory DIMMs on the Axon could be accessed only as one partition
when using file system drivers which are using the direct_access() method.
This patch enables for such file system drivers to access Axon's DDR2 memory
even if it is splitted in several partitions.

Signed-off-by: Maxim Shchetynin <maxim@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>