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>

Documentation/devicetree/bindings/timer/rockchip,rk3288-timer.txt → Documentation/devicetree/bindings/timer/rockchip,rk-timer.txt

-Rockchip rk3288 timer
+Rockchip rk timer
 
 Required properties:
-- compatible: shall be "rockchip,rk3288-timer"
+- compatible: shall be one of:
+  "rockchip,rk3288-timer" - for rk3066, rk3036, rk3188, rk322x, rk3288, rk3368
+  "rockchip,rk3399-timer" - for rk3399
 - reg: base address of the timer register starting with TIMERS CONTROL register
 - interrupts: should contain the interrupts for Timer0
 - clocks : must contain an entry for each entry in clock-names