MIPS: generic: Select MIPS_AUTO_PFN_OFFSET

Enable CONFIG_MIPS_AUTO_PFN_OFFSET for the generic platform, allowing
it to avoid wasted book-keeping for pages with addresses lower than the
physical base address of memory.

This has a minimal impact on kernel text size, with 64r6el_defconfig
gaining 0.1% in size as reported by bloat-o-meter:

  add/remove: 4/1 grow/shrink: 345/13 up/down: 9017/-392 (8625)
  Function                                     old     new   delta
  pcpu_setup_first_chunk                      1444    1780    +336
  pcpu_alloc_first_chunk                       864    1136    +272
  start_kernel                                1064    1288    +224
  initcall_blacklist                           224     372    +148
  try_fill_recv                               2088    2184     +96
  ...
  Total: Before=8457273, After=8465898, chg +0.10%

The gain for systems with large offsets to physical memory & the ability
to continue using generic kernels on such systems seems well worth this
small cost.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Suggested-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
Patchwork: https://patchwork.linux-mips.org/patch/20049/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org