serial: sh-sci: Document that serial aliases became optional

Serial aliases are optional since commit 7678f4c2

 ("serial:
sh-sci: Add support for dynamic instances").
Update the DT bindings to reflect this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>