dt-bindings: Clarify compatible property for rockchip timers

Make all properties description in form '"rockchip,<chip>-timer",
"rockchip,rk3288-timer"' for all chips found in linux kernel.

Suggested-by: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>