dt-bindings: mailbox: rockchip: add tx poll period property

Specify TX Done polling interval in milliseconds with the
"rockchip,txpoll-period-ms" property.

Signed-off-by: Frank Wang <frank.wang@rock-chips.com>
Change-Id: I91f25e6b5e6ef05cf90147b2e609c71c6ed61df7

Documentation/devicetree/bindings/mailbox/rockchip-mailbox.txt

@@ -17,6 +17,10 @@ Required properties:
  - #mbox-cells: Common mailbox binding property to identify the number
 	of cells required for the mailbox specifier. Should be 1
 
+Optional properties :
+
+ - rockchip,txpoll-period-ms: TX Done polling interval in milliseconds.
+
 Example:
 --------