rockchip: remove binman temporary files when cleaning

Binman mkimage entry generates temporary files so let's remove them
when calling `make clean`.

Fixes: 9b312e26 ("rockchip: Enable building a SPI ROM image on jerry")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Reported-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>