Staging: speakup: Fix allyesconfig build on mn10300

mn10300 uses an mmio-driven uart. This is not supported by speakup, so
prevent from enabling it.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>