clk: add devm_get_clk_from_child() API

Some driver is using this type of DT bindings for clock (more detail,
see ${LINUX}/Documentation/devicetree/bindings/sound/simple-card.txt).

	sound_soc {
		...
		cpu {
			clocks = <&xxx>;
			...
		};
		codec {
			clocks = <&xxx>;
			...
		};
	};

Current driver in this case uses of_clk_get() for each node, but there
is no devm_of_clk_get() today.
OTOH, the problem of having devm_of_clk_get() is that it encourages the
use of of_clk_get() when clk_get() is more desirable.

Thus, this patch adds new devm_get_clk_from_chile() which explicitly
reads as get a clock from a child node of this device.
By this function, we can also use this type of DT bindings

	sound_soc {
		clocks = <&xxx>, <&xxx>;
		clock-names = "cpu", "codec";
		clock-ranges;
		...
		cpu {
			...
		};
		codec {
			...
		};
	};

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[sboyd@codeurora.org: Rename subject to clk + add API]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/clk-devres.c

@@ -53,3 +53,24 @@ void devm_clk_put(struct device *dev, struct clk *clk)
 	WARN_ON(ret);
 }
 EXPORT_SYMBOL(devm_clk_put);
+
+struct clk *devm_get_clk_from_child(struct device *dev,
+				    struct device_node *np, const char *con_id)
+{
+	struct clk **ptr, *clk;
+
+	ptr = devres_alloc(devm_clk_release, sizeof(*ptr), GFP_KERNEL);
+	if (!ptr)
+		return ERR_PTR(-ENOMEM);
+
+	clk = of_clk_get_by_name(np, con_id);
+	if (!IS_ERR(clk)) {
+		*ptr = clk;
+		devres_add(dev, ptr);
+	} else {
+		devres_free(ptr);
+	}
+
+	return clk;
+}
+EXPORT_SYMBOL(devm_get_clk_from_child);