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>

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

@@ -56,6 +56,12 @@ Required properties:
     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).
+    On UARTs equipped with a Baud Rate Generator for External Clock (BRG)
+    (some SCIF and HSCIF), additional clocks may be specified:
+      - "brg_int" for the optional internal clock source for the frequency
+	divider (typically the (AXI or SHwy) bus clock),
+      - "scif_clk" for the optional external clock source for the frequency
+	divider (SCIF_CLK).
 
 Note: Each enabled SCIx UART should have an alias correctly numbered in the
 "aliases" node.