mips32: add gdb target description support

This commit is inspired by

    commit 1255b18f
    Author: Spencer Oliver <spen@spen-soft.co.uk>
    Date:   Fri Sep 13 09:44:36 2013 +0100

        armv7m: add gdb target description support

Change-Id: I75c3971fd0599d34ed49fb73975378b57f2a4af0
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
CC: Spencer Oliver <spen@spen-soft.co.uk>
CC: Oleksij Rempel <linux@rempel-privat.de>
CC: Paul Fertser <fercerpav@gmail.com>
CC: Gregory Fong <gregory.0xf0@gmail.com>
Reviewed-on: http://openocd.zylin.com/1972


Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins