astbmc: Add a system-id device tree property
astbmc: Add a system-id device tree property

The pel logging code requires a system-id device tree property. This
should be passed to us from Hostboot but at present we don't seem to
get one. This patch adds a default value of "unavailable" if it
doesn't exist.

Signed-off-by: Alistair Popple <e9adbcf81c8a36ccc628e09bc92d0e25e1c8e501@popple.id.au>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

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