RFC RFC: dm: Add I2C support

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(Note this is RFC since the uclass interface needs discussion and also because only sandbox is implemented so far. But I thought it best to get this out there as soon as I wrote it as it may influence the PMIC library, etc.)

This series is an initial attempt to add I2C support to driver model. It has become apparent that this is a high priority as it is widely used. It follows along to some extent from the SPI conversion.

Several changes are made from the original I2C implementations.

Firstly It is not necessary to specify the chip address with every call, since each chip knows its own address - it is stored in struct dm_i2c_chip which is attached to each chip on the I2C bus. However, this information *is* passed to the driver since I presume most drivers need it and it would be cumbersome to look up in every call.

Secondly there is no concept of a 'current' I2C bus so all associated logic is removed. With driver model i2c_set_bus_num() and i2c_get_bus_num() are not available. Since the chip device specifies both the bus and the chip address, there is no need for this concept. It also causes problems when one driver changes the current bus and forgets to change it back.

Thirdly initialisation is handled by driver model's normal probe() method on each device so there should be no need for i2c_init_all(), i2c_init(), i2c_init_board(), i2c_board_late_init() and board_i2c_init().

I2C muxes are not yet supported. To support these we will need to maintain state of the current mux settings to avoid resetting every mux every time. Probably we need to add a sandbox I2C mux driver to permit testing of this. This can probably be done later.

Platform data is not yet supported either, only device tree. The U_BOOT_I2C_MKENT_COMPLETE() and U_BOOT_I2C_ADAP_COMPLETE() macros are not used. Also struct i2c_adapter is not defined anymore. This will need to be addressed, perhaps as part of converting over a board that does not use device tree.

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This series is available at u-boot-dm/i2c-working.

Nothing jumped out at me. Heiko?