arm64: dts: rockchip: Add u-boot, spl-boot-order for ROCKPi N10

Add u-boot,spl-boot-order for ROCKPi N10, so-that it can able
to boot from eMMC and SDMMC in order.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>