bpf, doc: add bpf trees and tps to maintainers entry

i) Add the bpf and bpf-next trees to the maintainers entry
   so they can be found easily and picked up by test bots
   etc that would integrate all trees from maintainers file.
   Suggested by Stephen while integrating the trees into
   linux-next.

ii) Add the two headers defining BPF/XDP tracepoints to the
    list of files as well.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>