ARM: dts: rockchip: disable arm-global-timer for rk3188

The clocksource and the sched_clock provided by the arm_global_timer
are quite unstable because their rates depend on the cpu frequency.

On the other side, the arm_global_timer has a higher rating than the
rockchip_timer, it will be selected by default by the time framework
while we want to use the stable rockchip clocksource.

Let's disable the arm_global_timer in order to have the rockchip
clocksource selected by default.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>