dt-bindings: document the MPS2 timer bindings

This adds documentation of device tree bindings for the
timers found on ARM MPS2 platform.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>