ffmpeg: use the new BR2_ARM_CPU_ARM* options

Instead of using the Config.in options for each individual ARM core,
use the newly introduced BR2_ARM_CPU_ARM* options, which exist per ARM
architecture.

This commit is a blind conversion from the per-core options to the
per-architecture. It has the benefit of enabling the ARMv6
optimizations on all ARMv7-A cores, not only A5, A8, A9 and
A15. However, it doesn't fix the condition for
--enable-vfp/--disable-vfp for ARMv6 cores that don't have a FPU, it
is fixed in a follow-up commit.

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