external/opal-prd: Add asm directory for opal-prd link
external/opal-prd: Add asm directory for opal-prd link

Signed-off-by: Jeremy Kerr <68a658b44bbaefab3ef5cd231967195fc0ff9d69@ozlabs.org>

open-power/skiboot,shenki/skiboot,ddstreet/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,csmart/skiboot,ddstreet/skiboot,apopple/skiboot,mikey/skiboot,ddstreet/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,cyrilbur-ibm/skiboot,legoater/skiboot,legoater/skiboot,jk-ozlabs/skiboot,qemu/skiboot,apopple/skiboot,open-power/skiboot,jk-ozlabs/skiboot,legoater/skiboot,mikey/skiboot,shenki/skiboot,shenki/skiboot,cyrilbur-ibm/skiboot,open-power/skiboot,csmart/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,jk-ozlabs/skiboot,csmart/skiboot,apopple/skiboot,mikey/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,cyrilbur-ibm/skiboot,open-power/skiboot,shenki/skiboot