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: 4aa78300 ("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>