rockchip: block: add rkmtd class and drivers

Add rkmtd class and drivers to create a virtual block device
to transfer Rockchip boot block data to and from NAND with
block orientated tools like "ums" and "rockusb".

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