rockchip: clk: rk3188: Allow configuration of the armclk

The armclk starts in slow mode (24MHz) on the rk3188, which makes the whole
startup take a lot of time. We therefore want to at least move to the safe
600MHz value we can use with default pmic settings.
This is also the freqency the proprietary sdram-init leaves the cpu at.

For boards that have pmic control later in u-boot, we also add the option
to set the maximum frequency of 1.6GHz, if they so desire.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Simon Glass <sjg@chromium.org>