rockchip: Add support for the SPI image

The Rockchip boot ROM requires a particular file format for booting from SPI.
It consists of a 512-byte header encoded with RC4, some padding and then up
to 32KB of executable code in 2KB blocks, separated by 2KB empty blocks.

Add support to mkimage so that an SPL image (u-boot-spl-dtb.bin) can be
converted to this format. This allows booting from SPI flash on supported
machines.

Signed-off-by: Simon Glass <sjg@chromium.org>