stb: Obey secure-enabled in device tree
stb: Obey secure-enabled in device tree

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,stewart-ibm/skiboot,shenki/skiboot,open-power/skiboot,mikey/skiboot,shenki/skiboot,mikey/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot,stewart-ibm/skiboot,open-power/skiboot,open-power/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,stewart-ibm/skiboot,shenki/skiboot,qemu/skiboot,mikey/skiboot,open-power/skiboot,qemu/skiboot