clk: rockchip: reparent aclk_cpu_pre to the gpll

aclk_cpu_pre on the rk3188 can either be sourced from the armclk or the gpll.
To reduce complexity on apll changes caused by cpufreq, reparent it always
to the gpll source.

If really necessary it could be reparented back on a per board level using
the assigned-clocks mechanism.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>