blk: blkmap: Support mapping to device of any block size

At present if a device to map has a block size other than 512,
the blkmap map process just fails. There is no reason why we
can't just use the block size of the mapped device.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>