CLOCKSOURCE: mips-gic: Allow GIC clock to be specified in device-tree

As an alternative to the "clock-frequency" property, allow the GIC
timer operating clock to be specified in the device-tree instead.
This is useful on systems which use common clock or where the GIC
is not fixed to a particular frequency and is instead, for example,
derived from the CPU clock.

Signed-off-by: Andrew Bresticker <[e-mail]>
Cc: James Hogan <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Pawel Moll <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: Kumar Gala <[e-mail]>
Cc: Daniel Lezcano <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/9309/

Documentation/devicetree/bindings/interrupt-controller/mips-gic.txt

···
 Required properties for timer sub-node:
 - compatible : Should be "mti,gic-timer".
 - interrupts : Interrupt for the GIC local timer.
+
+Optional properties for timer sub-node:
+- clocks : GIC timer operating clock.
 - clock-frequency : Clock frequency at which the GIC timers operate.
+
+Note that one of clocks or clock-frequency must be specified.
 
 Example:
 

drivers/clocksource/mips-gic-timer.c

···
  *
  * Copyright (C) 2012 MIPS Technologies, Inc.  All rights reserved.
  */
+#include <linux/clk.h>
 #include <linux/clockchips.h>
 #include <linux/cpu.h>
 #include <linux/init.h>
···
 
 static void __init gic_clocksource_of_init(struct device_node *node)
 {
+	struct clk *clk;
+
 	if (WARN_ON(!gic_present || !node->parent ||
 		    !of_device_is_compatible(node->parent, "mti,gic")))
 		return;
 
-	if (of_property_read_u32(node, "clock-frequency", &gic_frequency)) {
+	clk = of_clk_get(node, 0);
+	if (!IS_ERR(clk)) {
+		gic_frequency = clk_get_rate(clk);
+		clk_put(clk);
+	} else if (of_property_read_u32(node, "clock-frequency",
+					&gic_frequency)) {
 		pr_err("GIC frequency not specified.\n");
 		return;
 	}