clk: return probe defer when DT clock not yet ready

At probe time, a clock device may not be ready when some other device
wants to use it.

This patch lets the functions clk_get/devm_clk_get return a probe defer
when the clock is defined in the DT but not yet available.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

drivers/clk/clkdev.c

@@ -167,6 +167,8 @@ struct clk *clk_get(struct device *dev, const char *con_id)
 		clk = of_clk_get_by_name(dev->of_node, con_id);
 		if (!IS_ERR(clk))
 			return clk;
+		if (PTR_ERR(clk) == -EPROBE_DEFER)
+			return clk;
 	}
 
 	return clk_get_sys(dev_id, con_id);