arch: remove BR2_ARCH_HAS_ATOMICS option

Now that BR2_ARCH_HAS_ATOMICS is no longer used anywhere, we can
remove it from arch/Config.in*, as well as from the documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>