eeprom: at24: remove the write-protect pin support

NVMEM framework is an interface for the at24 EEPROMs as well as for
other drivers, instead of passing the wp-gpios over the different
drivers each time, it would be better to pass it over the NVMEM
subsystem once and for all.

Removing the support for the write-protect pin after adding it to the
NVMEM subsystem.

Signed-off-by: Khouloud Touil <ktouil@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>