clk: Revise help text for clk_get_parent_rate()

The function returns the rate of the parent clock, the previous text
made no sense at all.

Fixes: 4aa78300a0 ("dm: clk: Define clk_get_parent_rate() for clk operations")
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

include/clk.h

@@ -444,7 +444,7 @@ 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_get_parent_rate() - Get rate of current clock's parent.
  * @clk:	A clock struct that was previously successfully requested by
  *		clk_request/get_by_*().
  *