kgdb,mips: Individual register get/set for mips

Implement the ability to individually get and set registers for kdb
and kgdb for mips.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
CC: linux-mips@linux-mips.org