[POWERPC] cpm_uart: OF-related fix for CPM1

This makes cpm uart able to work using OF-passed parameters
in case of CPM stuff (found on most mpc8xx reference and custom
boards). The idea is to keep ppc stuff working yet making it able to be
used for powerpc.

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>