clocksource: keystone: add bindings for keystone timer

* Device tree bindings for Texas instruments Keystone timer

This document provides bindings for the 64-bit timer in the KeyStone

architecture devices. The timer can be configured as a general-purpose 64-bit

timer, dual general-purpose 32-bit timers. When configured as dual 32-bit

timers, each half can operate in conjunction (chain mode) or independently

(unchained mode) of each other.

It is global timer is a free running up-counter and can generate interrupt

when the counter reaches preset counter values.

Documentation:

http://www.ti.com/lit/ug/sprugv5a/sprugv5a.pdf

Required properties:

- compatible : should be "ti,keystone-timer".

- reg : specifies base physical address and count of the registers.

- interrupts : interrupt generated by the timer.

- clocks : the clock feeding the timer clock.

Example:

timer@22f0000 {

	compatible = "ti,keystone-timer";

	reg = <0x022f0000 0x80>;

	interrupts = <GIC_SPI 110 IRQ_TYPE_EDGE_RISING>;

	clocks = <&clktimer15>;

};