dt-bindings: i2c: Use correct vendor prefix for Atmel

The "at," prefix was never correct for Atmel, so fix the few occurrences
that got it wrong. Use "atmel," instead.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Acked-by: Peter Rosin <peda@axentia.se>
[wsa: merged two patches into one, dropped 'trivial-devices' hunk which
      needs to go in seperately]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>