devtree: add binding documentation for sp804

The sp804 binding is already in use by several platforms. This adds missing
documentation for the binding and also extends the binding to handle some
additional possible interrupt configurations.
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/timer/arm,sp804.txt

+ARM sp804 Dual Timers
+---------------------------------------
+
+Required properties:
+- compatible: Should be "arm,sp804" & "arm,primecell"
+- interrupts: Should contain the list of Dual Timer interrupts. This is the
+	interrupt for timer 1 and timer 2. In the case of a single entry, it is
+	the combined interrupt or if "arm,sp804-has-irq" is present that
+	specifies which timer interrupt is connected.
+- reg: Should contain location and length for dual timer register.
+- clocks: clocks driving the dual timer hardware. This list should be 1 or 3
+	clocks.	With 3 clocks, the order is timer0 clock, timer1 clock,
+	apb_pclk. A single clock can also be specified if the same clock is
+	used for all clock inputs.
+
+Optional properties:
+- arm,sp804-has-irq = <#>: In the case of only 1 timer irq line connected, this
+	specifies if the irq connection is for timer 1 or timer 2. A value of 1
+	or 2 should be used.
+
+Example:
+
+	timer0: timer@fc800000 {
+		compatible = "arm,sp804", "arm,primecell";
+		reg = <0xfc800000 0x1000>;
+		interrupts = <0 0 4>, <0 1 4>;
+		clocks = <&timclk1 &timclk2 &pclk>;
+		clock-names = "timer1", "timer2", "apb_pclk";
+	};