GENERICIO: Allow "empty"/invalid generic IO
I2C: Allow 0-, 8- and 16-bit memory addresses for read/write
AT24CXX: Adapt to the changed I2C API
MAG3110: Adapt to the changed I2C API
PCF8563: Adapt to the changed I2C API
GENERICIO: const-correctness
GENERICIO: Add generic_io_is_valid()
PCF8574: Implement the PCF8574 I2C I/O expander