modules/admin-full: add support for configuring mirror port

This adds the LuCI support for setting a mirror port on AR8327 switches.
Patch provided by Colin Leitner <colin.leitner@gmail.com>

Ref: https://dev.openwrt.org/attachment/ticket/10202

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>