ARM: dts: Enable emmc and sdmmc on the rk3288-evb boards

This enables basic SD and eMMC support.  Things are not yet running at
the fastest speed and we don't have the regulators specified, but we
can at least use the eMMC and SD cards now.

A note:
* Though MMC DDR50 mode is partially supported in the dw_mmc
  rk3288-specific code in Addy's patch, Addy's patch doesn't add
  tuning support.  That means DDR50 mode is not reliable.  From the
  3288 TRM: "Tuning is required for other speed modes-such as
  DDR50-even though the output delay from the card is less than one
  cycle."  Thus, we don't enable MMC DDR50 mode in this patch.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>