arm: stm32: Disable ATAGs support

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

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>