serial: sh-sci: Update DT binding documentation for external clock input

serial: sh-sci: Update DT binding documentation for external clock input

Amend the DT bindings to include the optional external clock on
(H)SCI(F) and some SCIFA, where this pin can serve as a clock input,
depending on board wiring.

Clarify the use of the divided functional clock as a source for the
sampling clock.

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