I2C: Add real clock control

[omeh-a]

Currently the I2C driver has absolutely no idea how to set its timing. It depends on hard-coded constants to set clock dividers, and this means that the functionality of the I2C is now dependent on the system clocks being exactly what those hard-coded values are tuned for AND is fully insensitive to variations in device clocking.

This principally refers to the Meson I2C driver (as the only extant one), but this will be needed in general.

To solve this issue, a clock driver must be connected to I2C to allow it to detect the clock rate and tune itself OR the I2C must be given access to the relevant clocking hardware to set the divider values appropriately.