powerpc/axonram: Use only one block device major number

Axonram module registers one block device for each DDR2 DIMM found
on a system. This means that each DDR2 DIMM becomes its own block device
major number. This patch lets axonram module to register the only one
block device for all DDR2 DIMMs which also spares kernel resources.

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>