arm64: dts: rockchip: Add rktimer device node for rk3399

Add a 'rktimer' node in the device treee for the ARM64 rk3399 SoC.

Signed-off-by: Huang Tao <huangtao@rock-chips.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Stuebner <heiko@sntech.de>
Tested-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>