serial: sh-sci: Add device tree bindings documentation

Document the device tree bindings for the sci serial port devices.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

diff --git a/Documentation/devicetree/bindings/serial/renesas,sci-serial.txt b/Documentation/devicetree/bindings/serial/renesas,sci-serial.txt
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+* Renesas SH-Mobile Serial Communication Interface
+
+Required properties:
+
+  - compatible: Must contain one of the following:
+
+    - "renesas,scif-r8a7790" for R8A7790 (R-Car H2) SCIF compatible UART.
+    - "renesas,scifa-r8a7790" for R8A7790 (R-Car H2) SCIFA compatible UART.
+    - "renesas,scifb-r8a7790" for R8A7790 (R-Car H2) SCIFB compatible UART.
+    - "renesas,hscif-r8a7790" for R8A7790 (R-Car H2) HSCIF compatible UART.
+    - "renesas,scif-r8a7791" for R8A7791 (R-Car M2) SCIF compatible UART.
+    - "renesas,scifa-r8a7791" for R8A7791 (R-Car M2) SCIFA compatible UART.
+    - "renesas,scifb-r8a7791" for R8A7791 (R-Car M2) SCIFB compatible UART.
+    - "renesas,hscif-r8a7791" for R8A7791 (R-Car M2) HSCIF compatible UART.
+    - "renesas,scif" for generic SCIF compatible UART.
+    - "renesas,scifa" for generic SCIFA compatible UART.
+    - "renesas,scifb" for generic SCIFB compatible UART.
+    - "renesas,hscif" for generic HSCIF compatible UART.
+
+    When compatible with the generic version, nodes must list the
+    SoC-specific version corresponding to the platform first followed by the
+    generic version.
+
+  - reg: Base address and length of the I/O registers used by the UART.
+  - interrupts: Must contain an interrupt-specifier for the SCIx interrupt.
+
+  - clocks: Must contain a phandle and clock-specifier pair for each entry
+    in clock-names.
+  - clock-names: Must contain "sci_ick" for the SCIx UART interface clock.
+
+Note: Each enabled SCIx UART should have an alias correctly numbered in the
+"aliases" node.
+
+Example:
+       aliases {
+               serial0 = &scifa0;
+       };
+
+       scifa0: serial@e6c40000 {
+               compatible = "renesas,scifa-r8a7790", "renesas,scifa-generic";
+               reg = <0 0xe6c40000 0 64>;
+               interrupt-parent = <&gic>;
+               interrupts = <0 144 IRQ_TYPE_LEVEL_HIGH>;
+               clocks = <&mstp2_clks R8A7790_CLK_SCIFA0>;
+               clock-names = "sci_ick";
+       };