i2c: i2c-cdns: Use proper input frequency

This is needed to properly calculate i2c bus speed divisors.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>
Signed-off-by: Wojciech Tatarski <wtatarski@antmicro.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>