powerpc/dts: Use 'atmel' as at24 manufacturer for pdm360ng

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