gst-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.

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