ARM: dts: rockchip: support highspeed sd-cards for rk3188-radxarock

Add cap-sd-highspeed and cap-mmc-highspeed for rk3188-radxarock
board to make sd cards running faster.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>