dts: ca5: add the global timer for the A5

The Cortex A5 contains a global timer: add the appropriate device tree
node.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>