dm: clk: Define clk_get_parent_rate() for clk operations

This commit adds the clk_get_parent_rate() function, which is responsible
for getting the rate of parent clock.
Unfortunately, u-boot's DM support for getting parent is different
(the parent relationship is in udevice) than the one in Common Clock
Framework [CCF] in Linux.

To alleviate this problem - the clk_get_parent_rate() function has been
introduced to clk-uclass.c.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

include/clk.h

@@ -267,6 +267,15 @@ ulong clk_get_rate(struct clk *clk);
  */
 struct clk *clk_get_parent(struct clk *clk);
 
+/**
+ * clk_get_parent_rate() - Get parent of current clock rate.
+ *
+ * @clk:	A clock struct that was previously successfully requested by
+ *		clk_request/get_by_*().
+ * @return clock rate in Hz, or -ve error code.
+ */
+long long clk_get_parent_rate(struct clk *clk);
+
 /**
  * clk_set_rate() - Set current clock rate.
  *