arm: mediatek: Disable ATAGs support

These platforms never had to support an ATAGs-based Linux Kernel, so
remove the options.

Cc: Ryder Lee <ryder.lee@mediatek.com>
Cc: Weijie Gao <weijie.gao@mediatek.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
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I'm assuming, please correct me if I'm wrong.