iod: Enhance to support display of multiple values

At present the 'iod' command differs from 'md' in that it only shows a
single value. It is useful to see a dump of multiple values, particularly
when x86 peripherals contain register sets accessible via I/O ports.

Enhance the command to match md.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct multi-line comment format style]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>