rockchip: test: dm: add rkmtd test

Add Rockchip rkmtd test:
Create/attach/detach RKMTD device.
Send/read data with Rockchip boot block header.
Test that reusing the same label should work.
Basic test of 'rkmtd' commands.

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