dt-bindings: Document the Broadcom STB wake-up timer node

Document the binding for the Broadcom STB SoCs wake-up timer node
allowing the system to generate alarms and exit low power states.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/rtc/brcm,brcmstb-waketimer.txt

+Broadcom STB wake-up Timer
+
+The Broadcom STB wake-up timer provides a 27Mhz resolution timer, with the
+ability to wake up the system from low-power suspend/standby modes.
+
+Required properties:
+- compatible     : should contain "brcm,brcmstb-waketimer"
+- reg            : the register start and length for the WKTMR block
+- interrupts     : The TIMER interrupt
+- interrupt-parent: The phandle to the Always-On (AON) Power Management (PM) L2
+                    interrupt controller node
+- clocks	 : The phandle to the UPG fixed clock (27Mhz domain)
+
+Example:
+
+waketimer@f0411580 {
+	compatible = "brcm,brcmstb-waketimer";
+	reg = <0xf0411580 0x14>;
+	interrupts = <0x3>;
+	interrupt-parent = <&aon_pm_l2_intc>;
+	clocks = <&upg_fixed>;
+};