[switch-optimization]: wb_i2c_master: adding support for multiple i2c interfaces

Commit adds new WB register(IFS) that selects to which I2C interface master
should talk to. All other interfaces are then hold in idle state. IFS contains
also BUSY bit that is written only by host and marks that I2C Master is
currently in use and cannot be switched to another I2C interface. Host must
clear BUSY flag after the interaction with I2C interface is finished.