doc: Added MIPS target document

This document explains MIPS EJTAG operations
implementations within OpenOCD.

Change-Id: Ieaf01f8bc5a97d7b0f2c847bac5fbb2ccf8ba088
Signed-off-by: Drasko DRASKOVIC <drasko.draskovic@gmail.com>
Reviewed-on: http://openocd.zylin.com/904
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>