doc/device-tree/ibm, opal/firmware/version: clarify timeline of git-i…

…d vs version

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

doc/device-tree/ibm,opal/firmware.rst

@@ -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.