backfire: swconfig: Add cpu port index to help output. (backport of 20939)

Let swconfig provide the cpu port index in its help page. This is
needed as e.g. Atheros switches have their cpu port at port 0, not
port 5.

This could allow e.g. luci to get a rough overview of the layout of
the switch.

Signed-off-by: Jonas Gorski <jonas.gorski+openwrt@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/backfire@20948 3c298f89-4303-0410-b956-a3cf2f4a3e73