sparc64: Add generic interface for registering a dimm printing handler.

The way to do this varies by platform type and the exact memory
controller the cpu uses.

For Spitfire cpus we currently just use prom_getunumber() and hope
that works.

For Cheetah cpus we have a memory controller driver that can
compute this information.

Signed-off-by: David S. Miller <davem@davemloft.net>