arm: dart6ul: change compatible string for eeprom

The eeprom at address 0x50 is a BR24G04NUX-3TTR. It has a
4Kbit (512x8) capacity, change the compatible string to reflect this
fact.

Also, add an alias to easily refer to this eeprom with
fdt_path_offset() which will be in another commit.

Signed-off-by: Marc Ferland <ferlandm@amotus.ca>
Reviewed-by: Fabio Estevam <festevam@gmail.com>