dm: Add a new header for block devices

At present block devices are tied up with partitions. But not all block
devices have partitions within them. They are in fact separate concepts.

Create a separate blk.h header file for block devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>