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>

Documentation/devicetree/bindings/serial/renesas,sci-serial.txt

@@ -51,6 +51,11 @@ Required properties:
   - clocks: Must contain a phandle and clock-specifier pair for each entry
     in clock-names.
   - clock-names: Must contain "fck" for the SCIx UART functional clock.
+    Apart from the divided functional clock, there may be other possible
+    sources for the sampling clock, depending on SCIx variant.
+    On (H)SCI(F) and some SCIFA, an additional clock may be specified:
+      - "hsck" for the optional external clock input (on HSCIF),
+      - "sck" for the optional external clock input (on other variants).
 
 Note: Each enabled SCIx UART should have an alias correctly numbered in the
 "aliases" node.