documentaion: DT: allow a A5 compatible string in global timer

The global timer is present on the Cortex A5. Add a compatibility
string to the DT binding to allow a Cortex A5 global timer.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>