ARM: dts: consistently use 'atmel' as at24 manufacturer in cyclone5

Using 'at' or 'at24' as the <manufacturer> part of the compatible
string is now deprecated. Use a correct value: 'atmel,<model>'.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>