clk: fractional-divider: rename prate -> parent_rate

Rename function parameter to be more explicit what it is for. This also makes
it in align with struct clk_ops.

There is no functional change.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>