i2c: core: Provide generic definitions for bus frequencies

There are few maximum bus frequencies being used in the I²C core code.
Provide generic definitions for bus frequencies and use them in the core.

The drivers may use predefined constants where it is appropriate.
Some of them are already using these under slightly different names.
We will convert them later to use newly introduced defines.

Note, the name of modes are chosen to follow well established naming
scheme [1].

These definitions will also help to avoid typos in the numbers that
may lead to subtle errors.

[1]: https://en.wikipedia.org/wiki/I%C2%B2C#Differences_between_modes



Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>