eeprom: at24: Support custom device names for AT24 EEPROMs

By using the label property, a more descriptive name can be populated
for AT24 EEPROMs NVMEM device. Update the AT24 driver to check to see
if the label property is present and if so, use this as the name for
NVMEM device. Please note that when the 'label' property is present for
the AT24 EEPROM, we do not want the NVMEM driver to append the 'devid'
to the name and so the nvmem_config.id is initialised to
NVMEM_DEVID_NONE.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>