selftests/kexec: Enable secureboot tests for PowerPC

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>