arm64: add kernel config option to set securelevel when in Secure Boot mode

arm64: add kernel config option to set securelevel when in Secure Boot mode

Add a kernel configuration option to enable securelevel, to restrict
userspace's ability to modify the running kernel when UEFI Secure Boot is
enabled. Based on the x86 patch by Matthew Garrett.

Determine the state of Secure Boot in the EFI stub and pass this to the
kernel using the FDT.

Signed-off-by: Linn Crosetto <linn@hpe.com>
Gbp-Pq: Topic features/all/securelevel
Gbp-Pq: Name arm64-add-kernel-config-option-to-set-securelevel-wh.patch