DT: arm,gic-v3: Update the ITS size in the examples

Currently, the examples are using 2MB for the ITS size. Per the
specification (section 8.18 in ARM IHI 0069D), the ITS address map is
128KB.

Update the examples to match the specification.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>