m68k/defconfig: Enable Atari EtherNAT and EtherNEC Ethernet support

Enable support for Atari EtherNAT (SMC91X) and EtherNEC (NE2000)
Ethernet support in the Atari and multiplatform defconfig files.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>