powerpc/dts: Use 'atmel' as at24 manufacturer for kmcent2

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: Michael Ellerman <mpe@ellerman.id.au>