ARM: dts: rockchip: set dw_mmc max-freq 150Mhz

All of mmc controllers include SDMMC, SDIO0, SDIO1, and EMMC on RK3288
are limited to 150Mhz. It was mainly caused by two reasons:
- RK3288's IO pad(except DDR IO pad) is generic, which can only support
  the max of 150Mhz.
- Mmc controller was designed at 150Mhz, and the pressure test by IC team
  was based on this freequency point.

Signed-off-by: Addy Ke <addy.ke@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>