rockchip: rk356x-u-boot: Use relaxed u-boot,spl-boot-order

BootRom will try to load TPL+SPL from media in the following order:
- SPI NOR Flash
- SPI NAND Flash
- NAND Flash
- eMMC
- SDMMC

SPL will try to load FIT from media in the order defined in the device
tree u-boot,spl-boot-order property.

Change the default order to load FIT from to:
- same media as TPL+SPL
- SDMMC
- eMMC

Boards with strict load order requirements should override the
u-boot,spl-boot-order property in the board specific u-boot.dtsi.

Fixes: 42f67fb5 ("rockchip: rk3568: Fix boot device detection")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>