devicetree: cadence_ttc: Document binding for timer width

Modern TTC implementations can extend the timer width to 32 bit. This
feature is not self identifying so the driver needs to be made aware
via device tree.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/timer/cadence,ttc-timer.txt

@@ -6,6 +6,9 @@ Required properties:
 - interrupts : A list of 3 interrupts; one per timer channel.
 - clocks: phandle to the source clock
 
+Optional properties:
+- timer-width: Bit width of the timer, necessary if not 16.
+
 Example:
 
 ttc0: ttc0@f8001000 {
@@ -14,4 +17,5 @@ ttc0: ttc0@f8001000 {
 	compatible = "cdns,ttc";
 	reg = <0xF8001000 0x1000>;
 	clocks = <&cpu_clk 3>;
+	timer-width = <32>;
 };