MIPS: ralink: rt288x: select MIPS_AUTO_PFN_OFFSET

MIPS: ralink: rt288x: select MIPS_AUTO_PFN_OFFSET

RT288X systems may have a non-zero ramstart causing problems with memory
reservations and boot hangs, as well as messages like:
  Wasting 1048576 bytes for tracking 32768 unused pages

Both are alleviated by selecting MIPS_AUTO_PFN_OFFSET for such
platforms.

Tested on a Belkin F5D8235 v1 RT2880 device.

Link: https://lore.kernel.org/linux-mips/20180820233111.xww5232dxbuouf4n@pburton-laptop/
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>