Documentation: DT: omap_serial: document missing properties and add an example

The omap_serial.txt binding documentation lacks a number of properties
that are used in DTS files for platforms incorporating this peripheral.
Fix this by documenting the missing required and optional fields and
add an example.
Signed-off-by: Matt Porter <mporter@konsulko.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

Documentation/devicetree/bindings/serial/omap_serial.txt

@@ -4,7 +4,27 @@ Required properties:
 - compatible : should be "ti,omap2-uart" for OMAP2 controllers
 - compatible : should be "ti,omap3-uart" for OMAP3 controllers
 - compatible : should be "ti,omap4-uart" for OMAP4 controllers
+- reg : address and length of the register space
+- interrupts or interrupts-extended : Should contain the uart interrupt
+                                      specifier or both the interrupt
+                                      controller phandle and interrupt
+                                      specifier.
 - ti,hwmods : Must be "uart<n>", n being the instance number (1-based)
 
 Optional properties:
 - clock-frequency : frequency of the clock input to the UART
+- dmas : DMA specifier, consisting of a phandle to the DMA controller
+         node and a DMA channel number.
+- dma-names : "rx" for receive channel, "tx" for transmit channel.
+
+Example:
+
+                uart4: serial@49042000 {
+                        compatible = "ti,omap3-uart";
+                        reg = <0x49042000 0x400>;
+                        interrupts = <80>;
+                        dmas = <&sdma 81 &sdma 82>;
+                        dma-names = "tx", "rx";
+                        ti,hwmods = "uart4";
+                        clock-frequency = <48000000>;
+                };