rockchip: rk3588-orangepi-5-plus: 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: b51cf8bb ("board: rockchip: Add Xunlong Orange Pi 5 Plus")
Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>