rockchip: add u-boot-rockchip-spi.bin image for booting from SPI-NOR flash

This new image is similar to u-boot-rockchip.bin except that it's
destined to be flashed on SPI-NOR flashes.

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