ARM: at91/defconfig: at91_dt: remove ARM_AT91_ETHER

CONFIG_ARM_AT91_ETHER doesn't exist anymore, both drivers have been merged
in the macb driver.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>