powerpc/machdep: Drop include of seq_file.h

Drop the include of seq_file.h in machdep.h, replace it with a forward
declaration of struct seq_file, which is all that's required.

Add direct includes of seq_file.h to some files that were getting
seq_file.h via machdep.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241009051826.132805-1-mpe@ellerman.id.au