speakup: only build serialio when ISA is enabled

Drivers using serialio were already made available in Kconfig only under
the ISA condition.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20200804160659.7y76sdseow43lfms@function


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>