arc: dts: use 'atmel' as manufacturer for at24 in axs10x_mb

Using compatible strings without the <manufacturer> part for at24 is
deprecated since commit 6da28acf ("dt-bindings: at24: consistently
document the compatible property"). Use a correct 'atmel,<model>'
value.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>