dt-bindings: eeprom: at24: Add label property for AT24

Add a label property for the AT24 EEPROM to allow a custom name to be
used for identifying the EEPROM on a board. This is useful when there
is more than one EEPROM present.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>