rockchip: rk3188: Setup the armclk in spl

The armclk starts in slow mode (24MHz) on the rk3188, which results in U-Boot
startup taking a lot of time (U-Boot itself, but also the rc4 decoding done
in the bootrom).

With default pmic settings we can always reach a safe frequency of 600MHz
which is also the frequency the proprietary loader left the armclk at,
without needing access to the systems pmic.

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