clk: rockchip: save width in struct clk_fractional_divider

The ->mwidth and ->nwidth fields will be used by clk-fractional-divider when it
will be switched to rational base approximation algorithm.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>