arm: stm32: Disable ATAGs support

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

Cc: Kamil Lulko <kamil.lulko@gmail.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Vikas Manocha <vikas.manocha@st.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
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I'm assuming, please correct me if I'm wrong.