rockchip: generate idbloader.img content for u-boot-rockchip.bin with binman for ARM

idbloader.img content - currently created by way of Makefile - can be
created by binman directly.

So let's do that for Rockchip ARM platforms.

Cc: Quentin Schulz <foss+uboot@0leil.net>
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>