clk: add ARM syscon ICST device tree bindings

ARM System Controller ICST clocks

The ICS525 and ICS307 oscillators are produced by Integrated Devices

Technology (IDT). ARM integrated these oscillators deeply into their

reference designs by adding special control registers that manage such

oscillators to their system controllers.

The ARM system controller contains logic to serialize and initialize

an ICST clock request after a write to the 32 bit register at an offset

into the system controller. Furthermore, to even be able to alter one of

these frequencies, the system controller must first be unlocked by

writing a special token to another offset in the system controller.

The ICST oscillator must be provided inside a system controller node.

Required properties:

- lock-offset: the offset address into the system controller where the

  unlocking register is located

- vco-offset: the offset address into the system controller where the

  ICST control register is located (even 32 bit address)

- compatible: must be one of "arm,syscon-icst525" or "arm,syscon-icst307"

- #clock-cells: must be <0>

- clocks: parent clock, since the ICST needs a parent clock to derive its

  frequency from, this attribute is compulsory.

Example:

syscon: syscon@10000000 {

	compatible = "syscon";

	reg = <0x10000000 0x1000>;

	oscclk0: osc0@0c {

		compatible = "arm,syscon-icst307";

		#clock-cells = <0>;

		lock-offset = <0x20>;

		vco-offset = <0x0c>;

		clocks = <&xtal24mhz>;

	};

	(...)

};