drivers: timer: Introduce ARC timer driver

This commit introduces timer driver for ARC.

ARC timers are configured via ARC AUX registers so we use special
functions to access timer control registers.

This driver allows utilization of either timer0 or timer1
depending on which one is available in real hardware. Essentially
only existing timers should be mentioned in board's Device Tree
description.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

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

@@ -0,0 +1,24 @@
+ARC Timer
+
+Required properties:
+
+- compatible : should be "snps,arc-timer".
+- reg : Specifies timer ID, could be either 0 or 1.
+- clocks : Specifies clocks that drives the counter.
+
+Examples:
+
+timer@0 {
+	compatible = "snps,arc-timer";
+	clocks = <&core_clk>;
+	reg = <0>;
+};
+
+timer@1 {
+	compatible = "snps,arc-timer";
+	clocks = <&core_clk>;
+	reg = <1>;
+};
+
+NOTE: if you specify both timers, clocks always should be the same
+as each timer is driven by the same core clock.