arm: dts: rock960: Enable booting from eMMC when using SPL

This commits enables booting from eMMC when using SPL on 96Boards
Rock960 board by adding SDHCI to boot order. Since the SDHCI driver
already has the reloc flag, this works straightaway. While we are at it,
let's also include the common u-boot dtsi for rk3399.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>