serial: sh-sci: Update DT binding documentation for BRG support

Amend the DT bindings to include the optional clock sources for the Baud
Rate Generator for External Clock (BRG), as found on some SCIF variants
and on HSCIF.

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