MIPS: BPF: Move register definition to the BPF header

MIPS: BPF: Move register definition to the BPF header

The registers will be used by a subsequent patch introducing
ASM helpers so move them to a common header.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/10528/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>