powerpc/fadump: Fix sparse warnings

Sparse says:
  arch/powerpc/kernel/fadump.c:48:16: warning: symbol 'fadump_kobj' was not declared. Should it be static?
  arch/powerpc/kernel/fadump.c:55:27: warning: symbol 'crash_mrange_info' was not declared. Should it be static?
  arch/powerpc/kernel/fadump.c:61:27: warning: symbol 'reserved_mrange_info' was not declared. Should it be static?
  arch/powerpc/kernel/fadump.c:83:12: warning: symbol 'fadump_cma_init' was not declared. Should it be static?

And indeed none of them are used outside this file, they can all be made
static. Also fadump_kobj needs to be moved inside the ifdef where it's
used.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210421125402.1955013-1-mpe@ellerman.id.au