Kconfig: Drop CONFIG_CMD_PORTIO and associated command

This command is not used by any board. It also looks quite similar to the
'iod' and 'iow' commands which use the correct I/O macros.

Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>