rockchip: rk3588: Add default u-boot,spl-boot-order prop

Add a default u-boot,spl-boot-order prop to rk3588s-u-boot.dtsi and
remove the prop from board u-boot.dtsi files using the default value.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>