arch/arm: reorder NEON option

Stating whether to use the NEON extensions when it is optional in the
CPU really is completing the definition of the CPU we've just selected.

Move the ENABLE_NEON option just after the choice of the CPU variant,
and before any "software" option (ABI/VFP).

This will make sense in a moment, when we introduce a similar option for
enabling an optional VFP unit.

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