powerpc/powernv: only register log if OPAL supports doing so

Correct use of REGISTER/UNREGISTER is to check if the token exists
before calling. If we don't we get a "OPAL: Called with bad token 101 !"
error, which is harmless but may be alarming to some.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Acked-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>