drivers: i2c_cc32xx: Get clock frequency from DTS

The i2c_cc32xx driver used system clock frequency
as a base for I2C clock frequency calculation.
This commit corrects that by obtaining the needed value from DTS.

Please note, that for I2C devices the clock-frequency property
specifies SCK frequency, instead of frequency of the clock driving
peripheral. To solve that problem, a new property was added.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>