libbpf: Complete powerpc spec in bpf_tracing.h

Add definitions of PARM6 through PARM8 for powerpc architecture. Add
also a link to a functiona call sequence documentation for future reference.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-6-andrii@kernel.org