doc/device-tree/ibm, opal/firmware/version: clarify timeline of git-id vs version
@@ -22,8 +22,12 @@ The 'firmware' node under 'ibm,opal' lists system and OPAL firmware version.
property describes OPAL symbol start address and size.
``version``
- property describes OPAL version. Replaces 'git-id', so may
- not be present. On POWER9 and above, it is always present.
+ property describes OPAL version. It replaced a property named 'git-id' early
+ in the POWER8 lifecycle, so may not always be present. It will be absent on
+ IBM FSP based systems running firmware prior to FW840. The 'version' property
+ is present on all GA POWER8 BMC systems.
+
+ On POWER9 and above, it is always present.
``mi-version``
property describes Microcode Image. Only on IBM FSP systems.
