RK3399 use sdhci for eMMC and DW MMC for SD Card, and
spl will only try to boot from SDMMC if we don't specify
other boot device for spl-boot-order. So add sdhci and sdmmc
for spl-boot-order here.
Signed-off-by: Andy Yan <andy.yan@rock-chips.com> Reviewed-by: Kever Yang <kever.yang@rock-chips.com>