drivers/i2c: kconfig: Clean up I2C Kconfigs a bit

- Get rid of duplicate I2C dependencies, which show up in the
  documentation as e.g. 'I2C && I2C'. The 'source's in
  drivers/i2c/Kconfig are already within an 'if I2C' block.

- Factor out I2C_GPIO_<n> dependencies in drivers/i2c/Kconfig.gpio into
  'if I2C_GPIO_<n>' blocks.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
4 files changed