serial: sirf: add DT-binding document to describle detailed properties

we defined some special properties for hardware flow control, document them
for DT-binding.
Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/tty/serial/sirf-uart.txt

+* CSR SiRFprimaII/atlasVI Universal Synchronous Asynchronous Receiver/Transmitter *
+
+Required properties:
+- compatible : Should be "sirf,prima2-uart" or "sirf, prima2-usp-uart"
+- reg : Offset and length of the register set for the device
+- interrupts : Should contain uart interrupt
+- fifosize : Should define hardware rx/tx fifo size
+- clocks : Should contain uart clock number
+
+Optional properties:
+- sirf,uart-has-rtscts: we have hardware flow controller pins in hardware
+- rts-gpios: RTS pin for USP-based UART if sirf,uart-has-rtscts is true
+- cts-gpios: CTS pin for USP-based UART if sirf,uart-has-rtscts is true
+
+Example:
+
+uart0: uart@b0050000 {
+	cell-index = <0>;
+	compatible = "sirf,prima2-uart";
+	reg = <0xb0050000 0x1000>;
+	interrupts = <17>;
+	fifosize = <128>;
+	clocks = <&clks 13>;
+};
+
+On the board-specific dts, we can put rts-gpios and cts-gpios like
+
+usp@b0090000 {
+	compatible = "sirf,prima2-usp-uart";
+	sirf,uart-has-rtscts;
+	rts-gpios = <&gpio 15 0>;
+	cts-gpios = <&gpio 46 0>;
+};