I2C: zynq: Refactor frequency adjustment policy

I2C allows frequencies in the range from 0 to 100 kHz in standard and up
to 400 kHz in fast operating mode. Adjust the logic handling frequencies
accordingly, to generate a clock within spec and the selected I2C
operating mode.

Reported-by: Yashu Gosain <yashug@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Yashu Gosain <yashug@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>