i2c: designware: Convert to use struct i2c_timings

Instead of using custom variables and parser, convert the driver to use
the ones provided by I2C core.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Tested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>