mips32: add gdb target description support

This commit is inspired by

    commit 1255b18fc6
    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

src/target/mips32.h

@@ -66,6 +66,7 @@
 /* offsets into mips32 core register cache */
 enum {
 	MIPS32_PC = 37,
+	MIPS32_FIR = 71,
 	MIPS32NUMCOREREGS
 };