Makefile: Drop to handle rkspi image type

Makefile: Drop to handle rkspi image type

On rockchip platforms, SPI boot image creation is not
straightforward like MMC boot image creation where former
requires to specify tpl, spl in multimage format in mkimage,
and later simply do a concatenate mkimaged-tpl with spl.

On this note, let drop rkspi image type creation via kbuild
and let inform via rockchip.rst

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>