[POWERPC] Fix apparent typo "CONFIG_SERIAL_CPM_SMC".

  Replace an apparent typo of CONFIG_SERIAL_CPM_SMC with
CONFIG_SERIAL_CPM_SMC2.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>