axon_ram: add dax_operations support

Setup a dax_device to have the same lifetime as the axon_ram block
device and add a ->direct_access() method that is equivalent to
axon_ram_direct_access(). Once fs/dax.c has been converted to use
dax_operations the old axon_ram_direct_access() will be removed.

Reported-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

arch/powerpc/platforms/Kconfig

@@ -284,6 +284,7 @@ config CPM2
 config AXON_RAM
 	tristate "Axon DDR2 memory device driver"
 	depends on PPC_IBM_CELL_BLADE && BLOCK
+	select DAX
 	default m
 	help
 	  It registers one block device per Axon's DDR2 memory bank found