clk: rockchip: add special approximation to fix up fractional clk's jitter

>From Rockchips fractional divider description:
  3.1.9  Fractional divider usage
  To get specific frequency, clocks of I2S, SPDIF, UARTcan be generated by
  fractional divider. Generally you must set that denominator is 20 times
  larger than numerator to generate precise clock frequency. So the
  fractional divider applies only to generate low frequency clock like
  I2S, UART.

Therefore add a special approximation function that handles this
special requirement.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>