vt: add /dev/vcsu* to devices.txt

Also mention that the traditional devices provide glyph values whereas
/dev/vcsu* is unicode based.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>