rockchip: rk3588s-orangepi-5: Build SPI image

Enable building of the SPI image, u-boot-rockchip-spi.bin, now that we
know what bootsource id values BootRom use for SPI flash on RK3588.

Fixes: 28c5f941 ("board: rockchip: Add Xunlong Orange Pi 5")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>