rockchip: dm: prepare rkmtd UCLASS

Prepare a rkmtd UCLASS in use for writing Rockchip boot blocks
in combination with existing userspace tools and rockusb command.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>