ARM: dts: use 'atmel' as at24 manufacturer in am335x-boneblue

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

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Tony Lindgren <tony@atomide.com>