i2c_emul: init i2c config

Initialize the i2c config of the emulator based on the clock_frequency
property.

The emulator can be used without calling the i2c_configure function, but
the i2c_get_config would return an error. With this change, it is
possible to get the config prior to the i2c_configure call.

Signed-off-by: Dawid Niedzwiecki <dn@semihalf.com>