kernel: Enable CONFIG_ARM64_PAN to restrict kernel access to user space memory

kernel: Enable CONFIG_ARM64_PAN to restrict kernel access to user space memory

Enable the CONFIG_ARM64_PAN kernel security option, which leverages the
ARMv8.1 Privileged Access Never (PAN) extension to prevent the kernel
from directly accessing user space memory.

Instead, copy_to_user and similar functions must be used for data
transfer between kernel and user space. This feature is automatically
disabled at runtime on CPUs without PAN support, making it a no-op in
those cases.

Link: https://github.com/openwrt/openwrt/pull/16189
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>