clocksource: keystone: add bindings for keystone timer

This patch 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

Acked-by: Rob Herring <[e-mail]>
Acked-by: Santosh Shilimkar <[e-mail]>
Signed-off-by: Ivan Khoronzhuk <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>

Documentation/devicetree/bindings/timer/ti,keystone-timer.txt

···
+* 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>;
+};