binman: Add support for a rockchip-tpl entry

The rockchip-tpl entry can be used when an external TPL binary should be
used instead of the normal U-Boot TPL.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Eugen Hristev <eugen.hristev@collabora.com>