dt-bindings: timer: Add andestech atcpit100 timer

Add a document to describe Andestech atcpit100 timer and
binding information.

Signed-off-by: rick <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/timer/atcpit100_timer.txt

+Andestech ATCPIT100 timer
+------------------------------------------------------------------
+ATCPIT100 is a generic IP block from Andes Technology, embedded in
+Andestech AE3XX, AE250 platforms and other designs.
+
+This timer is a set of compact multi-function timers, which can be
+used as pulse width modulators (PWM) as well as simple timers.
+
+It supports up to 4 PIT channels. Each PIT channel is a
+multi-function timer and provide the following usage scenarios:
+One 32-bit timer
+Two 16-bit timers
+Four 8-bit timers
+One 16-bit PWM
+One 16-bit timer and one 8-bit PWM
+Two 8-bit timer and one 8-bit PWM
+
+Required properties:
+- compatible	: Should be "andestech,atcpit100"
+- reg		: Address and length of the register set
+- interrupts	: Reference to the timer interrupt
+- clock-frequency : The rate in HZ in input of the Andestech ATCPIT100 timer
+
+Examples:
+
+timer0: timer@f0400000 {
+	compatible = "andestech,atcpit100";
+	reg = <0xf0400000 0x1000>;
+	interrupts = <2 4>;
+	clock-frequency = <30000000>;
+}: