rockchip: clk: rk3188: change APLL to safe 600MHz

The commit 84a6a27a ("rockchip: rk3188: init CPU freq in clock
driver") changed ARM clock from 600MHz to 1600MHz. It made boot
unstable due to the fact that PMIC at the start generates insufficient
voltage for operation. See also: commit f4f57c58 ("rockchip:
rk3188: Setup the armclk in spl").

Fixes commit 84a6a27a ("rockchip: rk3188: init CPU freq in clock
driver").

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>