clk: add clk_set_parent()

Clocks may support multiple parents: this change introduces an
optional operation on the clk-uclass to set a clock's parent.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: David Wu <david.wu@rock-chips.com>

Series-changes: 2
- Fixed David's email address.

include/clk-uclass.h

@@ -77,6 +77,14 @@ struct clk_ops {
 	 * @return new rate, or -ve error code.
 	 */
 	ulong (*set_rate)(struct clk *clk, ulong rate);
+	/**
+	 * set_parent() - Set current clock parent
+	 *
+	 * @clk:        The clock to manipulate.
+	 * @parent:     New clock parent.
+	 * @return zero on success, or -ve error code.
+	 */
+	int (*set_parent)(struct clk *clk, struct clk *parent);
 	/**
 	 * enable() - Enable a clock.
 	 *