ARM: dts: rockchip: temporarily remove emmc hs200 speed from rk3288 minnie

The eMMC of the minnie Chromebook doesn't like our current method of
tuning and while there are solutions on the horizon, they still need
investigating. Other Chromebooks tune just fine with the emmc, so
simply disable tuning on Minnie for now.

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