DEVICETREE: Add bindings for the ATH79 PLL controllers

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/clock/qca,ath79-pll.txt

+Binding for Qualcomm Atheros AR7xxx/AR9XXX PLL controller
+
+The PPL controller provides the 3 main clocks of the SoC: CPU, DDR and AHB.
+
+Required Properties:
+- compatible: has to be "qca,<soctype>-cpu-intc" and one of the following
+  fallbacks:
+  - "qca,ar7100-pll"
+  - "qca,ar7240-pll"
+  - "qca,ar9130-pll"
+  - "qca,ar9330-pll"
+  - "qca,ar9340-pll"
+  - "qca,qca9550-pll"
+- reg: Base address and size of the controllers memory area
+- clock-names: Name of the input clock, has to be "ref"
+- clocks: phandle of the external reference clock
+- #clock-cells: has to be one
+
+Optional properties:
+- clock-output-names: should be "cpu", "ddr", "ahb"
+
+Example:
+
+	memory-controller@18050000 {
+		compatible = "qca,ar9132-ppl", "qca,ar9130-pll";
+		reg = <0x18050000 0x20>;
+
+		clock-names = "ref";
+		clocks = <&extosc>;
+
+		#clock-cells = <1>;
+		clock-output-names = "cpu", "ddr", "ahb";
+	};