rockchip: rk3588-nanopc-t6: 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: b0b80868 ("board: rockchip: add FriendlyElec NanoPC-T6 rk3588 board")
Signed-off-by: John Clark <inindev@gmail.com>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>