eeprom: at24: use SPDX identifier instead of GPL boiler-plate

Replace the GPL (or later) header with the SPDX identifier
for GPL-2.0+.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>