rockchip: rk3399: Add a default spl-boot-order prop

A lot of RK3399 boards use a u-boot,spl-boot-order of "same-as-spl",
&sdhci and &sdmmc.

Move this to rk3399-u-boot.dtsi and make this default for boards
currently missing a u-boot,spl-boot-order prop.

Before commit a7e69952 ("rockchip: spl: Cache boot source id for
later use") it was required to include the SPI flash node in the
u-boot,spl-boot-order prop to successfully load FIT from SPI flash.

The SPI flash node reference has been dropped from spl-boot-order from
pinebook-pro, roc-pc and rockpro64 now that "same-as-spl" also gets
resolved to the SPI flash node and loading FIT from SPI flash works
without having the node explicitly referenced in spl-boot-order prop.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>