libbpf: Complete mips spec in bpf_tracing.h

Add registers for PARM6 through PARM8. Add a link to an ABI. We don't
distinguish between O32, N32, and N64, so document that we assume N64
right now.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-5-andrii@kernel.org