dm: blk: Rename get_device() to blk_get_device_by_str()

The current name is too generic. The function returns a block device based
on a provided string. Rename it to aid searching and make its purpose
clearer. Also add a few comments.

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