linux-fslc: move aarch64 defconfig to match machine overrides

Mainline BSP does not provide 'mx8m' machine override anymore, it has been
replaced with more generic 'armv8a' value.

Move defconfig from mx8m to armv8a folder for fetcher to resolve search
path proper and pick up the kernel config file for mainline BSP for
aarch64 builds.

No content change has been introduced to defconfig, kernel commit sync
point is still kept at commit [a2f5ea9e314b].

Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>

diff --git a/recipes-kernel/linux/linux-fslc/mx8m/defconfig b/recipes-kernel/linux/linux-fslc/armv8a/defconfig
similarity index 100%
rename from recipes-kernel/linux/linux-fslc/mx8m/defconfig
rename to recipes-kernel/linux/linux-fslc/armv8a/defconfig