doc: dt: Add documentation for lpc3220-timer

Add DT bindings documentation for lpc3220-timer. This timer is
used as clocksource on many NXP platforms.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>