staging: speakup: Add unicode support to the speakup_dummy driver

This extends spk_io_ops with a synth_out_unicode which takes a u16 character
instead of just a byte, and extends spk_ttyio to implement it to emit
utf-8. spk_do_catch_up_unicode can then be introduced to benefit from
synth_out_unicode, and speakup_dummy made to use spk_do_catch_up_unicode instead
of spk_do_catch_up.

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