rockchip: Add support for the SD image

The Rockchip boot ROM requires a particular file format. It consists of
64KB of zeroes, a 512-byte header encoded with RC4, and then some executable
code.

Add support to mkimage so that an SPL image (u-boot-spl-dtb.bin) can be
converted to this format.

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