drivers: i2c: clean up Kconfig file

Several minor changes to clean up I2C Kconfig file
- align help text
- remove duplicate dependencies
- use unified naming for I2C port options
- replace outdated references to datasheet in help text
- add comments at the end of 'endif'

Change-Id: I452083feb29f40909e6e38324ff9d9961fc6bd07
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>