arm64: dts: juno: add GPU subsystem

Since we now have bindings for Mali Midgard GPUs, let's use them to
describe Juno's GPU subsystem, if only because we can. Juno sports a
Mali-T624 integrated behind an MMU-400 (as a gesture towards
virtualisation), in their own dedicated power domain with DVFS
controlled by the SCP.

CC: Liviu Dudau <liviu.dudau@arm.com>
CC: Sudeep Holla <sudeep.holla@arm.com>
CC: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>