armv8r64-extras/xen: Update fvp-baser-aemv8r64 revision

Update fvp-baser-aemv8r64 SRCREV to include two commits have been added
to the integration/mpu branch:
 * A fix to remove allocations from the first context switch.
 * A fix to ensure that guests correctly detect the active GIC.

Issue-Id: SCM-4387
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Id0389d7dfe5502ad6ef753c6d21761a5ece52098