UBUNTU: SAUCE: (lockdown) arm64: Allow locking down the kernel under EFI secure boot

UBUNTU: SAUCE: (lockdown) arm64: Allow locking down the kernel under EFI secure boot

Add support to arm64 for the CONFIG_LOCK_DOWN_IN_EFI_SECURE_BOOT
option. When enabled the lockdown LSM will be enabled with
maximum confidentiality when booted under EFI secure boot.

Based on an earlier patch by Linn Crosetto.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
[v2: ported to 5.7-rc1 and adapted to the new fdt parsing mechanism]
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>