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: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Tested-by: NStephen Warren <swarren@nvidia.com>