arch/Config.in.arm: only enable BR2_ARM_CPU_HAS_NEON for ARMv8 in 32bit mode

arch/Config.in.arm: only enable BR2_ARM_CPU_HAS_NEON for ARMv8 in 32bit mode

A number of packages use BR2_ARM_CPU_HAS_NEON to know if the target handles
aarch32 neon instructions, which is only true for ARMv8 cores when they are
running in 32bit mode.

Notice: These cores do support neon-like instructions using a different
encoding in 64bit mode (it is a required part of ARMv8, similar to the FPU).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>