clk: add API to enable clock

The most basic thing for clock is to enable it, but it is missing
in this uclass.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/clk/clk-uclass.c b/drivers/clk/clk-uclass.c
index 73dfd7d..19f6f07 100644
--- a/drivers/clk/clk-uclass.c
+++ b/drivers/clk/clk-uclass.c
@@ -32,6 +32,16 @@
 	return ops->set_rate(dev, rate);
 }
 
+int clk_enable(struct udevice *dev, int periph)
+{
+	struct clk_ops *ops = clk_get_ops(dev);
+
+	if (!ops->enable)
+		return -ENOSYS;
+
+	return ops->enable(dev, periph);
+}
+
 ulong clk_get_periph_rate(struct udevice *dev, int periph)
 {
 	struct clk_ops *ops = clk_get_ops(dev);