rockchip: dts: rk3399-evb: add sdmmc node

The sdmmc node is missing after the dts sync patch:
  167efc2c arm64: dts: rk3399: Sync v5.7-rc1 from Linux
But we still need it for boot from SD card, so add it.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>