ARM: dts: at91: use 'atmel' as at24 manufacturer for at91sam9260ek

Using compatible strings without the <manufacturer> part for at24 is
now deprecated. Use a correct 'atmel,<model>' value.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>