Existing test cases determine secureboot state using efi variable, which
is available only on x86 architecture.  Add support for determining
secureboot state using device tree property on PowerNV architecture.
Signed-off-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Reviewed-by: Nayna Jain <nayna@linux.ibm.com>
Tested-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>