ARM: dts: rockchip: Add rockchip timer node for rk3288

The rk3288 board uses the architected timers and these ones are shutdown when
the cpu is powered down. There is a need of a broadcast timer in this case to
ensure proper wakeup when the cpus are in sleep mode and a timer expires.

Add the timer node for the broadcast timer.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>