dt-bindings: Document rk3399 rk-timer bindings

Add a compatible string for rk3399 SoC because the timer is slightly different
from the older SoCs. So rename the file name from rockchip,rk3288-timer.txt to
rockchip,rk-timer.txt and clarify rockchip,rk3288-timer supported SoCs.

Signed-off-by: Huang Tao <huangtao@rock-chips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>