arm64: dts: rockchip: Add the broadcast-timer for RK3368 SoC

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.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>