rockchip: allow to build SPI images even without HAS_ROM option

This prepares for the creation of a u-boot-rockchip-spi.bin image
similar to u-boot-rockchip.bin to the exception it's destined for
SPI-NOR flashes instead of MMC storage medium.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>