arm64: dts: juno/fast models: using GIC macros instead of hardcoded values

There are macros that exist to indicate the GIC specific flags and
custom cell values as per the GIC DT bindings. It's used in most of the
places in these DTS files but not all. To maintain consistency, lets
use the macros at all the places.

Since DTC doesn't even warn is any cells are missing, it's very hard to
debug if that's the case. Changing to use macros avoids missing cells/
columns.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>