dt-bindings: clock: add description of LPC32xx clock controller

NXP LPC32xx SoC has a clocking and power control unit (CPC) as a part
of system control block (SCB). CPC is supplied by two external
oscillators and it manages core and most of peripheral
clocks, the change adds description of DT bindings for clock
controller found on LPC32xx SoC series.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>

Documentation/devicetree/bindings/clock/nxp,lpc3220-clk.txt

+NXP LPC32xx Clock Controller
+
+Required properties:
+- compatible: should be "nxp,lpc3220-clk"
+- reg:  should contain clock controller registers location and length
+- #clock-cells: must be 1, the cell holds id of a clock provided by the
+  clock controller
+- clocks: phandles of external oscillators, the list must contain one
+  32768 Hz oscillator and may have one optional high frequency oscillator
+- clock-names: list of external oscillator clock names, must contain
+  "xtal_32k" and may have optional "xtal"
+
+Examples:
+
+	/* System Control Block */
+	scb {
+		compatible = "simple-bus";
+		ranges = <0x0 0x040004000 0x00001000>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		clk: clock-controller@0 {
+			compatible = "nxp,lpc3220-clk";
+			reg = <0x00 0x114>;
+			#clock-cells = <1>;
+
+			clocks = <&xtal_32k>, <&xtal>;
+			clock-names = "xtal_32k", "xtal";
+		};
+	};