percpu: add missing per_cpu_ptr_to_phys() definition for UP

Commit 3b034b0d implemented
per_cpu_ptr_to_phys() but forgot to add UP definition.  Add UP
definition which is simple wrapper around __pa().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>