arm: aspeed: Disable ATAGs support

This platform never had to support an ATAGs-based Linux kernel, so
remove the support for it.

Cc: Ryan Chen <ryan_chen@aspeedtech.com>
Cc: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Cc: Aspeed BMC SW team <BMC-SW@aspeedtech.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
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I'm assuming, please correct me if I'm wrong.