drivers: i2c_mchp_xec: Enable slave mode support

Enable slave support for I2C device instances. Slave mode is
interrupt based, wheras master mode is still based on polling.

Remove ENI bit in master configuration since it is not needed for
master mode.

Signed-off-by: Francisco Munoz <francisco.munoz.ruiz@intel.com>
Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>