sh: Fix up cpu to node mapping in sysfs.

Currently cpu_to_node() is always 0 in the UP case, though
we do want to have the CPU association linked in under sysfs
even in the cases where we're only on a single CPU.

Fix this up, so we have the cpu0 link on all of the available
nodes that don't already have a CPU link of their own.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>