ARM: dts: rockchip: Add drive/sample clocks for rk3288 dw_mmc devices

The drive/sample clocks can be phase shifted.  The drive clock
could be used in a future patch to adjust hold times.  The sample
clock is used for tuning.

Signed-off-by: Alexandru M Stan <amstan@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>