MIPS: kprobes: Define regs_return_value()

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
To: ananth@in.ibm.com
To: anil.s.keshavamurthy@intel.com
To: davem@davemloft.net
To: masami.hiramatsu.pt@hitachi.com
Cc: linux-kernel@vger.kernel.org,
Cc: hschauhan@nulltrace.org
Patchwork: https://patchwork.linux-mips.org/patch/1529/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>