docs: List MB version dependency for Juno FWU as known issue

Change-Id: Ib37215ca4c9b515e54054290952eed5034582ba4
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

docs/change-log.rst

@@ -442,6 +442,11 @@ Known Issues
      relating to the hdlcd driver failing to suspend will be emitted on the
      Linux terminal.
 
+   - arm/juno: The firmware update use-cases do not work with motherboard
+     firmware version < v1.5.0 (the reset reason is not preserved). The Linaro
+     18.04 release has MB v1.4.9. The MB v1.5.0 is available in Linaro 18.10
+     release.
+
    - mediatek/mt6795: This platform does not build in this release
 
 Trusted Firmware-A - version 2.0