misc: eeprom: at24: support pm_runtime control

Although in the most platforms, the power of eeprom are alway
on, some platforms disable the eeprom power in order to meet
low power request. This patch add the pm_runtime ops to control
power to support all platforms.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
[Bartosz: rebased on top of current at24/for-next]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>