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>;
	};
	(...)
};